Next Greater Element III
<p>Given a positive integer <code>n</code>, find <em>the smallest integer which has exactly the same digits existing in the integer</em> <code>n</code> <em>and is greater in value than</em> <code>n</code>. If no such positive integer exists, return <code>-1</code>.</p>

<p><strong>Note</strong> that the returned integer should fit in <strong>32-bit integer</strong>, if there is a valid answer but it does not fit in <strong>32-bit integer</strong>, return <code>-1</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> n = 12
<strong>Output:</strong> 21
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> n = 21
<strong>Output:</strong> -1
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 2<sup>31</sup> - 1</code></li>
</ul>
Math; Two Pointers; String
class Solution {
public:
    int nextGreaterElement(int n) {
        string s = to_string(n);
        n = s.size();
        int i = n - 2, j = n - 1;
        for (; i >= 0 && s[i] >= s[i + 1]; --i)
            ;
        if (i < 0) return -1;
        for (; s[i] >= s[j]; --j)
            ;
        swap(s[i], s[j]);
        reverse(s.begin() + i + 1, s.end());
        long ans = stol(s);
        return ans > INT_MAX ? -1 : ans;
    }
};