556. Next Greater Element III

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 n and 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：

1、从后往前找，找到第一个相邻的逆序对num[i] < num[i+1]

2、从后往前找，找到第一个大于num[i]的数，num[j]

3、交换num[i]和num[j]两个数

4、将i+1~num.length-1之间的数反转

上述方法同样可以用来求所有数字的排列组合。

程序如下所示；
 

class Solution {
    public int nextGreaterElement(int n) {
        String s = String.valueOf(n);
        char[] ch = s.toCharArray();
        if (ch.length <= 1){
            return -1;
        }
        int i = 0;
        for (i = ch.length - 2; i >= 0; -- i){
            if (ch[i+1] > ch[i]){
                break;
            }
        }
        if (i == -1){
            return -1;
        }
        int j = ch.length - 1;
        while (ch[j] <= ch[i]) {
            j --;
        }
        char tmp = ch[i];
        ch[i] = ch[j];
        ch[j] = tmp;
        int l = i + 1, r = ch.length - 1;
        while (l < r){
            tmp = ch[l];
            ch[l++] = ch[r];
            ch[r--] = tmp;
        }
        long res = Long.parseLong(new String(ch));
        return (res > Integer.MAX_VALUE)?-1:(int)res;
    }
}