題目

Implement strStr().

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

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().

Example 1:

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

Example 2:

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

Example 3:

Input: haystack = "", needle = ""
Output: 0

思路

兩個指標方別指向頭及 needle長度的地方，從頭開始檢查 substr，找到目標就回傳 index，否則回傳 -1。

Solution

/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function(haystack, needle) {
    if (needle.length == 0) return 0
    
    let l = 0
    let r = needle.length - 1
    
    while(r < haystack.length) {
        if (haystack.substr(l, needle.length) == needle) {
            return l
        }
        l++
        r++
    }
    return -1
};

Complexity

• Time complexity : O(n)
• Space complexity : O(1)