[Leetcode] 556. Next Greater Element III 解题报告

题目：

Given a positive 32-bit integer n, you need to find the smallest 32-bit integer which has exactly the same digits existing in the integer nand is greater in value than n. If no such positive 32-bit integer exists, you need to return -1.

Example 1:

Input: 12
Output: 21

Example 2:

Input: 21
Output: -1

思路：

首先将正整数n转换为string，然后求string的next_permutation，然后将next_permutation的结果重新转化为int并返回即可。需要注意的是正整数溢出以及nex_permutation的返回结果小于n的情况，此时按照题目要求，都需要返回-1。

代码：

class Solution {
public:
    int nextGreaterElement(int n) {
        if (n <= 9) {
            return -1;
        }
        string s = to_string(n);
        for (int i = s.length() - 1; i >= 1; --i) {
            if (s[i] > s[i - 1]) {  // the first increase pair
                // find the first characer that is bigger than s[i - 1] from right to left
                for (int k = s.length() - 1; k >= i; --k) {
                    if (s[k] > s[i - 1]) {
                        swap(s[i - 1], s[k]);
                        break;
                    }
                }
                sort(s.begin() + i, s.end());
                long long ret = stoll(s);
                return ret > INT_MAX ? -1 : static_cast<int>(ret);
            }
        }
        return -1;
    }
};