Use stacks.
Push one letter by letter.
If stack.top() == current letter, pop the stack.
else push the letter in the stack.

#include<bits/stdc++.h>
using namespace std;
stack<char> S;
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    string s;
    cin >> s;
    int len = s.length();
    for(int i = 0; i < len;i++)
    {
        char c;
        if(S.empty())
            c = 0;
        else
            c = S.top();
        if(c == s[i])
            S.pop();
        else
            S.push(c);
    }
    string ans = "";
    len = S.size();
    for(int i = 0; i < len;i++)
    {
        char c = S.top();
        ans += c;
        S.pop();
    }
    reverse(ans.begin(),ans.end());
    cout << ans;
}

For Time limit exceeded:
If you get time limit exceeded, it must mean that you're trying to modify the
input string (i.e. trying to delete i-th chars). Try creating a new OUTPUT string
without modifying the INPUT string.

Note: if you're using a high language such as java or C#, try using the StringBuilder
object to build your output string.

For Wrong Answer(WA):
If you get WA, try some of these test cases:

Input: "dddd"
Output: ""

Input: "ddddd"
Output: "d"

Input: "abba"
Output: ""

Input: "abbbbac"
Output: "c"

Input: "wliisddsmmleeddw"
Output: ""

Input: "avcbbffcv"
Output: "a"

Hope this helps.

Edited by author 12.07.2012 00:47

#define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <cmath>
#include <algorithm>
#include <set>
#include <vector>
#include <string>
#include <cstdlib>

using namespace std;

void optimization() {
     cin.tie(nullptr);
     ios_base::sync_with_stdio(false);
}


int main() {
     optimization();

     string s;
     int k = -1;
     cin >> s;
     char c[200000];

     for (char & i : c) {
          i = ' ';
     }

     for (char i : s) {
          if (k > -1) {
               if (c[k] == i) {
                    c[k] = ' ';
                    k--;
               } else {
                    k++;
                    c[k] = i;
                    continue;
               }
          }
          else {
               k++;
               c[k] = i;
          }
     }
     for (char i : c) {
          if (i != ' ') {
               cout << i;
          }
     }

     return 0;
}

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

const ll MAX = 1e5+10;
const ll ZERO = 0;

#define endl '\n'
#define dokme(x) cout << x ;  return(0);
#define migmig ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);

stack < char > stk;

int main(){
    migmig;
    stk.push('.');
    char c[200001];
    cin >> c;
    int n=strlen(c);
    for (int i =0; i < n ; i ++ ){
        if (stk.top()!=c[i]){
            stk.push(c[i]);
        }
        else{
            stk.pop();
        }
    }
    n=stk.size()-1;
    memset(c,0,sizeof(c));
    for (int i = 0 ; i<n ; i ++ ){
        c[n-i-1]=stk.top();
        stk.pop();
    }
    cout << c;
}

#include <iostream>
#include <cstdlib>
#include <deque>

using namespace std;

int main()
{
    int h;
    char s[200001];
    scanf("%s", &s);
    int n=strlen(s);
    deque<char> v (s, s + n  );

    if(v.size()==2  && v[0]==v[1] ) return 0;
    b: h=2;
    for(int i=0; i<v.size()-1 ;i++)
          if(v[i] == v[i+1])
        {
            v.erase(v.begin() + i);
            v.erase(v.begin() + i);
            h=1;
        }

    if(h==1) goto b;

    for(int i=0; i<v.size(); i++)
        cout << v[i];

        return 0;
}

#include<iostream>
#include<string.h>
#include<deque>
#include<stdio.h>
using namespace std;
int main()
{
    string node;
    cin >> node;
    int one,two;
    one = 0;
    two = 1;
    int pre = 0;
    for(int i = 0;i < node.size();i++)
    {
        if(node[i] == '!')
        continue;
        if(node[i] == node[i+1])
        {
            node[i] = node[i+1] = '!';
            one = pre;
            if(i+2 < node.size())
            two = i+2;
            while(one> 0 && two < node.size())
            {
                while(one > 0 && node[one] == '!')
                one--;
                if(node[one] == node[two] )
                {
                    node[one] = node[two] = '!';
                    one--;two++;
                }
                else
                break;
            }
        }
        else
        pre = i;

    }

   for(int i = 0;i < node.size();i++)
   {
       if(node[i] != '!')
       cout << node[i];
   }

}

For test you can use this string:
hkjgnnngkikipipekkfkfgg

Correct result is:
hkjgngkikipipefkf

using System;

namespace pain
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();

            char[] ss = new char[s.Length];

            for (int i = 0; i < s.Length; i++)
                ss[i] = s[i];
            for (int i = 0; i < s.Length - 1;)
            {

                if (ss[i] == ss[i + 1])
                {

                    if (i - 1 >= 0)
                    {
                        int n = i + 2;
                        b:
                        if (n <= s.Length - 1)
                        {
                            if (ss[n] != ' ')
                            {
                                ss[i] = ss[n];
                                ss[n] = ' ';
                                ss[i + 1] = ' ';
                            }
                            else
                            {
                                n++;
                                goto b;
                            }
                        }
                        else
                            break;

                    }
                    else
                    {
                        ss[i] = ' ';
                        ss[i + 1] = ' ';
                    }
                    if (i >= 1)
                        i--;
                    else
                        i++;

                }
                else
                    i++;
            }

            for (int i = 0; i < s.Length; i++)
                if(ss[i] != ' ')
                    Console.Write(ss[i]);

            //Console.Write(st2(s));
            Console.ReadLine();
        }

        public static string st1(string s, ref int n)
        {
            if (s.Length - 1 > n)
            {
                if (s[n] == s[n + 1])
                {
                    s = s.Remove(n, 2);
                    if (n >= 2)
                        n -= 2;
                    else
                        n--;
                }
                n++;
                s = st1(s, ref n);
            }
            return s;
        }
        public static string st2(string s)
        {
            for (int i = 0; i < s.Length - 1; i++)
            {
                if (s[i] == s[i + 1])
                {
                    s = s.Remove(i, 2);
                    if (i >= 2)
                        i -= 2;
                }
            }
            return s;
        }
        public static string st3(string s)
        {
            int n = 0;
            int nt = s.Length - 1;

            string[] ss = new string[nt];

            for (int i = 0; i < nt; i++)
            {
                bool d = false;
                for (int j = i; j < nt; j++)
                {
                    if (ss[i] == s[j].ToString())
                    {
                        d = true;
                    }
                }
                if (!d)
                {
                    ss[n] = s[i].ToString();
                    n++;
                }
            }

            n = 0;

            while (nt != 0)
            {
                nt = s.Length;
                for (int i = 0; i < ss.Length; i++)
                {
                    s = s.Replace(ss[i] + ss[i], "");
                }
                if (nt == s.Length)
                    break;
            }
            return s;
        }
    }
}

Edited by author 02.04.2019 03:06

Why Wring answer?
import java.util.Scanner;

public class T1654 {
    public static char[] arr;
    public static String out = "";
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner sc = new Scanner(System.in);
        String inp = sc.next();
        sc.close();
        arr = new char[inp.length()];
        arr = inp.toCharArray();
        format();
        if(out != "") {
            System.out.println(out);
        } else {
            System.out.println("");
        }

    }

    public static void format() {
        boolean isFormat = false;
        int l = arr.length;
        for(int i = 0; i< l-1; i++) {
            if(arr[i] == arr[i+1]) {
                arr[i] = 0;
                arr[i+1] = 0;
                isFormat = true;
                } else if(arr[i] != arr[i+1] & arr[i] != 0){
                out += arr[i];
            }
        }
        out+= arr[arr.length-1];
        if(isFormat == true) {
            arr = new char[out.length()];
            arr = out.toCharArray();
            out = "";
            format();
        }
    }

}

  import random
  letters = ['a' ,'b' ,'c' ,'d' ,'e' ,'f' ,'g' ,'h' ,'i' ,'j' ,'k' ,'l' ,'m' ,'n' ,'o' ,'p' ,'q' ,'r' ,'s' ,'t' ,'u' ,'v' ,'w' ,'x' ,'y' ,'z']
  example = 'xitsa'
  while len(example) < 199999:
      letter = random.choice(letters)
      point = random.randrange(0, len(example))
      example = example[:point] + letter + letter + example[point:]
  print(example)

I thought that stack is the best thing in this case but I've seen 0.015 and 0.001 sec . I've got 0.046

char[200001] gives perfect time if you will check repeats while string is inputting

i used deque. cout got me TLE.using printf save me.

      if(st.front()!=s[i])
        {
            st.emplace_front(s[i]);
        }
        else
            st.pop_front();


for(it=st.end()-1; it>=st.begin(); it--)
    {
        printf("%c",*it);
    }

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Iterator;
import java.util.Stack;


public class problem1654 {

    public static void main(String[] args) throws IOException {
    BufferedReader inp = new BufferedReader(new InputStreamReader(System.in));
    String str=inp.readLine();
    Stack<Character> st=new Stack<Character>();
    st.push(str.charAt(0));
    String s="";
    for(int i=1;i<str.length();i++){
    if(!st.empty()&&st.peek()==str.charAt(i))
        st.pop();
    else{
    st.push(str.charAt(i));

    }
    }



while(!st.empty()){
    s=s+st.pop();
}
int n=s.length();
for(int i=0;i<n;i++){
    System.out.print(s.charAt(n-i-1));


    }

    }}

// Harry Poter
#include <stdio.h>
#include <iostream>
#include <stdlib.h>
#include <string.h>
using namespace std;
int main()
{
    char line[200001], w2[200001];
    cin.getline(line, sizeof(line));
    int t = 0, i;

    for(i = 0; i < strlen(line); i++){
            if (line[i] == line[i + 1]) {
                i++;
A:    if (line[i + 1] != line[i + 2]) {
        if (t != 0 && w2[t - 1] == line[i + 1]) {t--; i += 1; goto A;}
                }
            }
    else {w2[t++] = line[i];}
        }
    w2[t] = '\0';
    printf("%s", w2);
}

The test is "abcdeedcbaabcdeedcba"(without quotes).
The answer should be ""(empty string).

hey if one else to know print the anser plese say me..in the test #4 i have a empty string, but the judge say me "wa" in test #4.... why??? help me please.

#include <bits/stdc++.h>
using namespace std;

int main(){
    string cad,res;
             cin>>cad;
             res = "";
    int tam = cad.length();
    int i,j;
        i=0; j=1;

    int boolean[tam]={0};

    while(j<tam){
        if(cad[i]==cad[j]){
            boolean[i] = 1;
            boolean[j] = 1;

            if(i-1>=0){
                i-=1;
                j+=1;
            }else{
                i=j+1;
                j+=2;
            }
        }else{
            i=j;
            j+=1;
        }
    }

    for(int i=0;i<tam;i++){
         if(boolean[i]==0)
             res+=cad[i];
     }
     cout<<res;
    return 0;
}

Can any body tell me whai is in the test case 5...??? My code seems right but everytime it says WA#5...

import java.util.Scanner;
import java.util.Stack;




public class CliperMessage_1654_Stack {
    public static void main(String[] arg1) {
        Scanner sc=new Scanner(System.in);
        StringBuilder sb=new StringBuilder();
        Stack<Character> stc=new Stack<>();
        sb.append(sc.nextLine());
        if(sb.length()<=200000){
            for(int i=0;i<sb.length();i++){
                if(stc.isEmpty()||(stc.peek()!=sb.charAt(i)))
                    stc.push(sb.charAt(i));
                else
                    if((stc.peek()==sb.charAt(i))) stc.pop();

            }

            StringBuilder sbn=new StringBuilder();
            int j=0;
            while(!stc.empty()) sbn.insert(j++, ""+stc.pop());
            System.out.println(sbn.reverse());
        }
    }
}