Maximum Swap LeetCode solution in C++

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class Solution {
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 public:
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  int maximumSwap(int num) {
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    string st_n = to_string(num);
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    int maxNum = -1, maxIdx = -1;
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    int leftidx = -1, rightidx = -1;
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    for (int i = st_n.size() - 1; i >= 0; i--) {
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      if (st_n[i] > maxNum) {
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        maxNum = st_n[i];
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        maxIdx = i;
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        continue;
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      }
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      if (st_n[i] < maxNum) {
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        leftidx = i;
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        rightidx = maxIdx;
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      }
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    }
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    if (leftidx == -1) return num;
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    swap(st_n[leftidx], st_n[rightidx]);
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    return stoi(st_n);
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  }
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};