关注 每天一道编程题 专栏，一起学习进步。

题目

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.

Example 2:

Input: num = 9996
Output: 9999
Explanation: Changing the last digit 6 to 9 results in the maximum number.

Example 3:

Input: num = 9999
Output: 9999
Explanation: It is better not to apply any change.

Constraints:

1 <= num <= 10^4
num's digits are 6 or 9.

分析

题意：给一个正整数，只包含9和6，仅改变其中一个数字，使其最大。
很简单，从高到低判断是否为9即可，如果是，则判断下一位，如果不是，则改成9，并返回。

涉及到字符串和整型的转换。

解答

class Solution {
    public int maximum69Number (int num) {
        String s = String.valueOf(num);
        return Integer.parseInt(s.replaceFirst("6","9"));
    }
}

replaceFirst使用的是正则匹配，速度慢。


其他答案

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