# 0670. Maximum Swap

<https://leetcode.com/problems/maximum-swap>

## Description

You are given an integer `num`. You can swap two digits at most once to get the maximum valued number.

Return *the maximum valued number you can get*.

**Example 1:**

```
**Input:** num = 2736
**Output:** 7236
**Explanation:** Swap the number 2 and the number 7.
```

**Example 2:**

```
**Input:** num = 9973
**Output:** 9973
**Explanation:** No swap.
```

**Constraints:**

* `0 <= num <= 108`

## ac

```java
class Solution {
    public int maximumSwap(int num) {
        char[] chars = String.valueOf(num).toCharArray();
        int len = chars.length;
        int[] lastIndexOfDigit = new int[10];
        for (int i = 0; i < len; i++) {
            int digit = chars[i] - '0';
            lastIndexOfDigit[digit] = i;
        }
        
        for (int i = 0; i < len; i++) {
            int currDigit = chars[i] - '0';
            // Try to find a larger digit after this index i.
            for (int d = 9; d > currDigit; d--) {
                if (lastIndexOfDigit[d] > i) {
                    chars[i] = chars[lastIndexOfDigit[d]];
                    chars[lastIndexOfDigit[d]] = (char)('0' + currDigit);
                    return Integer.parseInt(String.valueOf(chars));
                }
            }
        }
        
        return num;
    }
}

// O(N * 9) time - N is len(num), O(N) space
```