Implement strStr() leetcode

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

就是给一个target 看target里面是否包含另外给定的字符

假设target长度m 匹配长度n, 对target每个长度为n的substring都检查一次 时间O((m- n)* n)= O(m*n)

public class Solution {
    public int strStr(String haystack, String needle) {
        if (haystack == null || needle == null || needle.length() == 0) {
            return 0;
        }
        if (needle.length() > haystack.length()) {
            return -1;
        }
        for (int i = 0; i <= haystack.length() - needle.length(); i++) {
            boolean res = true;
            for (int j = 0; j < needle.length(); j++) {
                if (haystack.charAt(i + j) != needle.charAt(j)) {
                    res = false;
                    break;
                }
            }
            if (res == true) {
                return i;
            }
        }
        return -1;
    }
}