LeetCode | Implement strStr()

题目

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

分析

KMP算法

代码

public class ImplementStrStr {
	public String strStr(String haystack, String needle) {
		if (haystack == null || needle == null) {
			return null;
		}
		if (needle.length() == 0) {
			return haystack;
		}
		if (haystack.length() == 0) {
			return null;
		}
		int M = haystack.length();
		int N = needle.length();

		int[] next = new int[N];
		next[0] = -1;
		int j = 0;
		int k = -1;
		while (j < N - 1) {
			if (k == -1 || needle.charAt(j) == needle.charAt(k)) {
				++j;
				++k;
				next[j] = k;
			} else {
				k = next[k];
			}
		}

		int i = 0;
		j = 0;
		while (i < M) {
			if (j == -1 || haystack.charAt(i) == needle.charAt(j)) {
				++i;
				++j;
			} else {
				j = next[j];
			}
			if (j == N) {
				return haystack.substring(i - N);
			}
		}

		return null;
	}
}