Codeforces Round #531 (Div. 3) D - Balanced Ternary String

从前往后把0换够n/3;把1换够n/3;从后往前换2；
从前往后变小，从后往前变大

`#include <bits/stdc++.h>

const int N = 300300;
const long long mod = 1e9 + 7;

using namespace std;

int n;
int c[4];
string s;

int main()
{

 ios_base::sync_with_stdio(0);//c++快速输入可和scanf达到一个层次的速度



        cin >> n >> s;
        for(int i = 0; i < n; i++){
                c[s[i] - '0'] += 1;
        }
        for(int i = 0; i < n && c[0] < n / 3; i++){//保证0的位数等于n/3 
                if(c[s[i] - '0'] <= n / 3){//大于n/3后向下运算 
                        continue;
                }
                if(s[i] != '0'){//如果为1或2 
                        c[s[i] - '0'] -= 1;//减去1个s[i] 
                        s[i] = '0';
                        c[s[i] - '0'] += 1;//0位加一个 
                }
        }
        for(int i = 0; i < n && c[1] < n / 3; i++){//保证1的位数等于n/3 
                if(c[s[i] - '0'] <= n / 3){
                        continue;
                }
                if(s[i] == '2'){
                        c[s[i] - '0'] -= 1;
                        s[i] = '1';
                        c[s[i] - '0'] += 1;
                }
        }
        for(int i = n - 1; i >= 0 && c[2] < n / 3; i--){//保证2的位数等于n/3 
                if(c[s[i] - '0'] <= n / 3){
                        continue;
                }
                if(s[i] != '2'){//1或0匀给2 
                        c[s[i] - '0'] -= 1;
                        s[i] = '2';
                        c[s[i] - '0'] += 1;
                }
        }
        for(int i = n - 1; i >= 0 && c[1] < n / 3; i--){
                if(c[s[i] - '0'] <= n / 3){
                        continue;
                }
                if(s[i] != '1'){//0或2匀给1 
                        c[s[i] - '0'] -= 1;
                        s[i] = '1';
                        c[s[i] - '0'] += 1;
                }
        }
        cout << s << "\n";
}`                             `