LeetCode 670. Maximum Swap

原题链接在这里：https://leetcode.com/problems/maximum-swap/

题目：

Given a non-negative integer, you could swap two digits at most once to get the maximum valued number. Return the maximum valued number you could get.

Example 1:

Input: 2736
Output: 7236
Explanation: Swap the number 2 and the number 7. 

Example 2:

Input: 9973
Output: 9973
Explanation: No swap. 

Note:

1. The given number is in the range [0, 108]

题解：

想要组成最大的数字，就是要把尽量开头的digit换成后面比他大的最大digit. 若这个最大digit有重复, 去最右侧的那个.

It is about how to get the index of largest digit after current one.

所以先把么个digit出现的last index记录下来.

Iterating num, check from max = 9 to check if max last occurance index is after i. If yes, swap.

Time Complexity: O(n). n是原有num digits的位数.

Space: O(n).

AC Java:

class Solution {
    public int maximumSwap(int num) {
        char [] digits = Integer.toString(num).toCharArray();
        
        int [] lastInd = new int[10];
        for(int i = 0; i<digits.length; i++){
            lastInd[digits[i]-'0'] = i;
        }
        
        for(int i = 0; i<digits.length; i++){
            for(int k = 9; k>digits[i]-'0'; k--){
                if(lastInd[k] > i){
                    char temp = digits[i];
                    digits[i] = digits[lastInd[k]];
                    digits[lastInd[k]] = temp;
                    return Integer.valueOf(new String(digits));
                }
            }
        }
        return num;
    }
}

类似Remove K Digits.