<p>You are given an integer <code>num</code>. You can swap two digits at most once to get the maximum valued number.</p>

<p>Return <em>the maximum valued number you can get</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> num = 2736
<strong>Output:</strong> 7236
<strong>Explanation:</strong> Swap the number 2 and the number 7.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> num = 9973
<strong>Output:</strong> 9973
<strong>Explanation:</strong> No swap.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= num &lt;= 10<sup>8</sup></code></li>
</ul>

class Solution {
public:
    int maximumSwap(int num) {
        if (num == 0) return 0;
        string s = to_string(num);
        char largest = -1;
        vector<int> prefix(s.size());
        for (int i = s.size() - 1; i >= 0; i--) {
            if (s[i] > largest) {
                largest = s[i];
                prefix[i] = i;
            } else {
                prefix[i] = prefix[i + 1];
            }
        }
        for (int i = 0; i < s.size(); i++) {
            if (s[i] < s[prefix[i]]) {
                swap(s[i], s[prefix[i]]);
                break;
            }
        }
        return atoi(s.c_str());
    }
};
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