LeetCode:Implement strStr()

Implement strStr()

Total Accepted: 109580  Total Submissions: 435297  Difficulty: Easy

Implement strStr().

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

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  Two Pointers String

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思路：

KMP算法。

c++ code:

class Solution {
public:
    int strStr(string haystack, string needle) {
        
        int i = 0;
        int j = 0;
        int sLen = haystack.length();
        int pLen = needle.length();
        int next[pLen];
        getNext(needle, next);
        while (i < sLen && j < pLen) {
            //①如果j = -1，或者当前字符匹配成功（即S[i] == P[j]），都令i++，j++  
            if (j == -1 || haystack[i] == needle[j]) {
                i++;
                j++;
            } else {
                //②如果j != -1，且当前字符匹配失败（即S[i] != P[j]），则令 i 不变，j = next[j]  
                //next[j]即为j所对应的next值  
                j = next[j];
            }
        }
        if (j == pLen)
            return i - j;
        else
            return -1;
        
    }
    
    void getNext(string p, int next[]) {
        int pLen = p.length();
        int k = -1;
        int j = 0;
        next[0] = -1;
        while (j < pLen) {
            //p[k]表示前缀，p[j]表示后缀
            if (k == -1 || p[k] == p[j]) {
                k++;
                j++;
                next[j] = k;
            } else {
                k = next[k];
            }  
        }  
    }
    
};