670. Maximum Swap

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.

分析：
先找到每个digit出现的最后位置。
然后从左到右，看是否有9到当前digit的另一个digit在当前digit的后面出现过，如果出现过，果断换。

class Solution {
    public int maximumSwap(int num) {
        char[] digits = Integer.toString(num).toCharArray();
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < digits.length; i++) {
            // if there are digits with the same value, we choose the last one.
            // 27367 -> 77362
            map.put(digits[i] - '0', i);
        }
        
        for (int i = 0; i < digits.length; i++) {
            for (int k = 9; k > digits[i] - '0'; k--) {
                if (map.getOrDefault(k, - 1) > i) {
                    char tmp = digits[i];
                    digits[i] = digits[map.get(k)];
                    digits[map.get(k)] = tmp;
                    return Integer.valueOf(new String(digits));
                }
            }
        }
        
        return num;
    }
}