28. Implement strStr() 以及KMP算法的实现

28. Implement strStr()

题目描述

Implement strStr().

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

代码实现

这里需要注意的是不需要把母字符串全部迭代一次，只需要到还剩下子串长度的索引地方。

代码一：

class Solution {
public:
int strStr(string haystack, string needle) {
int h_len = haystack.length();
int n_len = needle.length();
int rel = -1;

if(h_len < n_len) return rel;

if(haystack == needle || needle == "") return 0;

for(int i = 0; i <= h_len - n_len; i++) {
for(int k = 0, j = i; k < n_len && j < h_len; k++, j++) {
if(haystack[j] != needle[k])
break;
else if(k == n_len - 1)
return i;
}
}

return rel;
}
};

代码二：把上面的代码精简一下。

class Solution {
public:
int strStr(string haystack, string needle) {
int m = haystack.length(), n = needle.length();
if (!n) return 0;
for (int i = 0; i < m - n + 1; i++) {
int j = 0;
for (; j < n; j++)
if (haystack[i + j] != needle[j])
break;
if (j == n) return i;
}
return -1;
}
};

当然在寻找子串方面，KMP是最经典的算法。

使用KMP寻找子串可以得到：

class Solution {
public:
int strStr(string haystack, string needle) {
int m = haystack.length(), n = needle.length();
if (!n) return 0;
vector<int> lps = kmpProcess(needle);
for (int i = 0, j = 0; i < m; ) {
if (haystack[i] == needle[j]) {
i++;
j++;
}
if (j == n) return i - j;
if (i < m && haystack[i] != needle[j]) {
if (j) j = lps[j - 1];
else i++;
}
}
return -1;
}

private:
vector<int> kmpProcess(string& needle) {
int n = needle.length();
vector<int> lps(n, 0);
for (int i = 1, len = 0; i < n; ) {
if (needle[i] == needle[len])
lps[i++] = ++len;
else if (len) len = lps[len - 1];
else lps[i++] = 0;
}
return lps;
}
};

KMP算法的介绍：

http://jakeboxer.com/blog/2009/12/13/the-knuth-morris-pratt-algorithm-in-my-own-words/

http://www.geeksforgeeks.org/searching-for-patterns-set-2-kmp-algorithm/

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