leetcode-implement-strstr

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题目大意

　　https://leetcode.com/problems/implement-strstr/

　　实现字符串匹配判断

题目分析

　　利用kmp算法，具体原理可以看下我之前的一篇博客

代码

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public class Solution {
    
    public int strStr(String haystack, String needle) {
        char[] txt = haystack.toCharArray();
        char[] pat = needle.toCharArray();
        int n = txt.length;
        int m = pat.length;
        if (n < m) {
            return -1;
        }
        if (n == 0) {
            return 0;
        }
        if (m == 0) {
            return 0;
        }
        int[] lps = new int[m];
        // build lps array
        int preLen = 0;
        int i = 1;
        while (i < m) {
            if (pat[preLen] == pat[i]) {
                lps[i] = preLen + 1;
                i++;
                preLen++;
            } else {
                if (preLen == 0) {
                    lps[i] = 0;
                    i++;
                } else {
                    preLen = lps[preLen - 1];
                }
            }
        }
        // kmp
        i = 0;
        int j = 0;
        while (i < n) {
            if (txt[i] == pat[j]) {
                i++;
                j++;
            }
            if (j == m) {
                return i - j;
            } else if (i < n && txt[i] != pat[j]) {
                if (j > 0) {
                    j = lps[j - 1];
                } else {
                    i++;
                }
            }
        }
        return -1;
    }
}