28. Implement strStr()

Posted 2021-01-11 ruruozhenhao

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().

 

AC code:

this code is low effective.

class Solution {
public:
    int strStr(string haystack, string needle) {
        int len1 = haystack.length();
        int len2 = needle.length();
        if (len2 == 0) {
            return 0;
        }
        for (int i = 0; i < len1; ++i) {
            int index = i;
            int j = 0;
            while (haystack[index] == needle[j] && index < len1 && j < len2) {
                index++;
                j++;
            }
            if (j == len2) {
                return i;
            }
        }
        return -1;
    }
};

Runtime: 812 ms, faster than 3.41% of C++ online submissions for Implement strStr().

 

Only have a little change, but the effective have a big exaltation.

class Solution {
public:
    int strStr(string haystack, string needle) {
        int len1 = haystack.length();
        int len2 = needle.length();
        if (len2 == 0) {
            return 0;
        }
　　　　　// i < len1 - len2 
        for (int i = 0; i < len1-len2+1; ++i) {
            int index = i;
            int j = 0;
            while (haystack[index] == needle[j] && index < len1 && j < len2) {
                index++;
                j++;
            }
            if (j == len2) {
                return i;
            }
        }
        return -1;
    }
};

Runtime: 8 ms, faster than 37.69% of C++ online submissions for Implement strStr().

But this way is a brute-force algorithm, too. may be this question want you to use brute-force.

The high effective algorithm is to use KMP algorithm.

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;
    }
};

Runtime: 8 ms, faster than 37.69% of C++ online submissions for Implement strStr().