slow:

class Solution {
public:
    int maximumSwap(int num) {
        string s = to_string(num);
        int pos[10] = { -1 };
        for (int i = s.length() - 1; i >= 0; i--) {
            if (i > pos[s[i]-‘0‘])
                pos[s[i]-‘0‘] = i;
        }
        for (int i = 0; i < s.length(); i++) {
            
 
for (int j = 9; j > s[i]-‘0‘; j--)
                if (pos[j] > i) {
                    swap(s[pos[j]], s[i]);
                    return stoi(s);
                }
        }
        return num;
    }
};

faster:

class Solution {
public:
    int maximumSwap(int num) {
        string s = to_string(num);
        
 
int x = -1, y = -1, _max = s.length() - 1;
        for (int i = s.length() - 2; i >= 0; i--) {
            if (s[i] > s[_max]) {
                _max = i;
            }
            else if (s[i] < s[_max]) {
                x = i;
                y = _max;
            }
        }
        if (x >= 0
 
) {
            swap(s[x], s[y]);
        }
        return stoi(s);
    }
};

670. Maximum Swap