leetcode 1323. Maximum 69 Number（最大的69数字）

You are given a positive integer num consisting only of digits 6 and 9.

Return the maximum number you can get by changing at most one digit (6 becomes 9, and 9 becomes 6).

Example 1:

Input: num = 9669
Output: 9969
Explanation:
Changing the first digit results in 6669.
Changing the second digit results in 9969.
Changing the third digit results in 9699.
Changing the fourth digit results in 9666.
The maximum number is 9969.

69数字是指只含数字6和9的数字。
只允许换一位，换成6或9，变成最大的数字。

思路：

因为只有2个数字，所以就简单化了。
从高位到低位，遇到第一个6时换成9.

public int maximum69Number (int num) {
    char[] arr = Integer.toString(num).toCharArray();
    
    for(int i = 0; i < arr.length; i++) {
        if(arr[i] == '6') {
            arr[i] = '9';
            break;
        }
    }
    return Integer.parseInt(new String(arr));
}