Remove All Adjacent Duplicates In String.cpp

// Runtime: 44 ms, faster than 28.17% of C++ online submissions for Remove All Adjacent Duplicates In String.
// Memory Usage: 11.5 MB, less than 100.00% of C++ online submissions for Remove All Adjacent Duplicates In String.
#include <iostream>
#include <string>
using namespace std;

class Solution
{
public:
    string removeDuplicates(string S)
    {
        int nRemoved = 0;

        do
        {
            nRemoved = 0;
            for (int i = 0; i < S.size() - 1;)
            {
                if (S[i] == S[i + 1])
                {
                    // remove from i, length 2
                    //  cout << S.size() << ", " << i << endl;
                    S.erase(i, 2);
                    i--;
                    nRemoved++;
                }
                else
                {
                    i++;
                }
            }
        } while (nRemoved > 0 && S.size() > 0);

        return S;
    }
};

// Two pointer:
// https://leetcode.com/problems/remove-all-adjacent-duplicates-in-string/discuss/294893/JavaC%2B%2BPython-Two-Pointers-and-Stack-Solution
// Runtime: 20 ms, faster than 82.26% of C++ online submissions for Remove All Adjacent Duplicates In String.
// Memory Usage: 11.8 MB, less than 100.00% of C++ online submissions for Remove All Adjacent Duplicates In String.

class Solution
{
public:
    string removeDuplicates(string S)
    {
        int isrc, idst;
        for (isrc = 0, idst = 0; isrc < S.size(); isrc++, idst++)
        {
            S[idst] = S[isrc];
            if (idst > 0 && S[idst] == S[idst - 1])
            {
                idst -= 2;
            }
        }
        return S.substr(0, idst);
    }
};

// Stack
// https://leetcode.com/problems/remove-all-adjacent-duplicates-in-string/discuss/294893/JavaC%2B%2BPython-Two-Pointers-and-Stack-Solution
// Runtime: 16 ms, faster than 90.84% of C++ online submissions for Remove All Adjacent Duplicates In String.
// Memory Usage: 12.6 MB, less than 100.00% of C++ online submissions for Remove All Adjacent Duplicates In String.

class Solution
{
public:
    string removeDuplicates(string S)
    {
        string ans = "";
        for (char c : S)
        {
            if (ans.size() > 0 && c == ans.back())
            {
                ans.pop_back();
            }
            else
            {
                ans.push_back(c);
            }
        }
        return ans;
    }
};