19.1.30 [LeetCode 28] Implement strStr()

Implement strStr().

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

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1

Clarification:

What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

题意

实现找子串函数

题解

class Solution {
public:
    int* findNext(string P) {
        int i, k;
        int m = P.length();
        int *next = new int[m];
        next[0] = -1;
        i = 0; k = -1;
        while (i < m - 1) {
            while (k >= 0 && P[k] != P[i])
                k = next[k];
            i++, k++;
            if (P[k] == P[i])
                next[i] = next[k];
            else next[i] = k;
        }
        return next;
    }
    int strStr(string haystack, string needle) {
        if (needle == "")return 0;
        int *next = findNext(needle);
        int p1 = 0, p2 = 0;
        int l1 = haystack.length(), l2 = needle.length();
        while (p1 < l1&&p2 < l2) {
            if (haystack[p1] == needle[p2]) {
                p1++, p2++;
                continue;
            }
            p2 = next[p2];
            if (p2 == -1) {
                p1++; p2 = 0;
            }
        }
        if (p2 != l2)return -1;
        return p1 - l2;
    }
};

说到找子串就是kmp啦