字符串

1.力扣344-反转字符串
第一个和倒数第一个交换，第二个和倒数第二个交换

class Solution {
    public void reverseString(char[] s) {
        for(int i = 0, j = s.length - 1; i < j; i ++, j --){
            char t = s[i];
            s[i] = s[j];
            s[j] = t;
        }
    }
}

2.力扣541-反转字符串2

class Solution {
    public String reverseStr(String s, int k) {
        char[] ch = s.toCharArray();
        for(int i = 0; i < s.length(); i += 2 * k){
            int start = i;
            int end = Math.min(s.length() - 1, start + k - 1);
            while(start < end){
                char c = ch[start];
                ch[start] = ch[end];
                ch[end] = c;
                start ++;
                end --;
            }
        }
        return new String(ch);
    }
}

3.力扣28-找出字符串中第一个匹配项的下标（KMP算法）
！！！最主要的就是找到最长相等前后缀，前缀表

class Solution {
    public int strStr(String haystack, String needle) {
        int[] next = new int[needle.length() + 1];
        for(int j = 2, i = 0; j <= needle.length(); j ++){
            while(i > 0 && needle.charAt(i) != needle.charAt(j - 1)) i = next[i];
            if(needle.charAt(i) == needle.charAt(j - 1)) i ++;
            next[j] = i;
        }
        for(int i = 0, j = 0; i < haystack.length(); i ++){
            while(j > 0 && haystack.charAt(i) != needle.charAt(j)) j = next[j];
            if(haystack.charAt(i) == needle.charAt(j)) j ++;
            if(j == needle.length()){
                return i - j + 1;
            }  
        }
        return -1;
    }
}

4.力扣459-重复的子字符串-kmp算法

class Solution {
    public boolean repeatedSubstringPattern(String s) {
        int l = s.length();
        int[] next = new int[l + 1];
        for(int i = 0, j = 2; j <= l; j ++){
            while(i > 0 && s.charAt(i) != s.charAt(j - 1)) i = next[i];
            if(s.charAt(i) == s.charAt(j - 1)) i ++;
            next[j] = i;
        }
        if(next[l] > 0 && l % (l - next[l]) == 0) return true;
        else return false;
    }
}