424. Longest Repeating Character Replacemen

yPhantom 2019年10月23日 31次浏览

Given a string s that consists of only uppercase English letters, you can perform at most k operations on that string.

In one operation, you can choose any character of the string and change it to any other uppercase English character.

Find the length of the longest sub-string containing all repeating letters you can get after performing the above operations.

Note: Both the string's length and k will not exceed 104.

Example 1:

Input:
s = "ABAB", k = 2

Output:
4

Explanation:
Replace the two 'A's with two 'B's or vice versa.

Example 2:

Input:
s = "AABABBA", k = 1

Output:
4

Explanation:
Replace the one 'A' in the middle with 'B' and form "AABBBBA".
The substring "BBBB" has the longest repeating letters, which is 4.

Solution

窗口法。

我们假定一个left和一个right两个左右指针形成一个窗口,那么窗口的长度为j - i + 1,按照题意,如果这个窗口替换x个字符能达到窗口中全是重复的,那么只要x <= k就行了,然后将窗口从左往右遍历。

窗口法比较关键的问题是找到left与right的条件。在本题中,我们通过保存每个字符出现的次数,然后利用x <= k这个条件即可。

class Solution {
    public int characterReplacement(String s, int k) {
        int[] count = new int[26];
        int left = 0;
        int n = s.length();
        int maxLen = 0;
        int maxCount = 0;
        int result = 0;
        for (int right = 0; right < n; right++) {
            maxCount = Math.max(maxCount, ++count[s.charAt(right) - 'A']);
            while(right < n && right - left + 1 - maxCount <= k) {
                maxCount = Math.max(maxCount, ++count[s.charAt(right++) - 'A']);
            }
            result = Math.max(result, right - left + 1);
        }
        return result - 1;
    }
}