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：
可以理解成全排列的下一个。小心一个坑是，下一个排列必须在正整数范围内。

class Solution
    public int nextGreaterElement(int n) {
        String num = n + "";  
        char[] charArray = num.toCharArray();  
        if(num.length() < 2) 
            return -1;  
        int index = num.length() - 2;  
        for(; index >= 0; index --){  
            if(num.charAt(index) < num.charAt(index + 1)) 
                break;  
        }  
        if(index == -1) 
            return -1;  
        int swapIndex = index + 1;  
        for(;swapIndex < num.length();swapIndex ++){  
            if(num.charAt(swapIndex) <= num.charAt(index)) 
                break;  
        }  
        swapIndex --;  
        char temp = charArray[index];  
        charArray[index] = charArray[swapIndex];  
        charArray[swapIndex] = temp;  
        Arrays.sort(charArray,index + 1, charArray.length);  
        int result = 0;  
        for(char ch : charArray) {
            result = result * 10 + (ch - '0');
            if(result > Integer.MAX_VALUE / 10) 
                return -1;
        }  

        return