leetcode670+交换数字一次，获得最大值，暴力

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https://leetcode.com/problems/maximum-swap/description/

class Solution {
public:
    int maximumSwap(int num) {
        string a = to_string(num);
        int res = num;
        for(int i=0; i<a.size(); i++){
            for(int j=i+1; j<a.size(); j++){
                swap(a[i], a[j]);
                res = max(res, stoi(a));
                swap(a[i], a[j]);
            }
        }
        return res;
    }
};