题目描述
Implement
strStr()
.Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
例子
例子 1
Input: haystack = “hello”, needle = “ll” Output: 2
例子 2
Input: haystack = “aaaaa”, needle = “bba” Output: -1
例子 3
Input: Output: Explanation
Follow Up
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’sindexOf()
.
Constraints
haystack
andneedle
consist only of lowercase English characters.
解题思路
这道题思路比较清晰,通过两个指针来解决。其中一个指针 i
遍历 haystack
字符串,另一个指针 j
从 i
开始遍历 needle
的长度,判断每一位是否相同,只要其中一位不同就跳出中间循环将 i
移至下一位。代码如下:
#include <string>
class Solution {
public:
int strStr(std::string haystack, std::string needle) {
if (needle.empty()) return 0;
if (haystack.length() < needle.length()) return -1;
for (int i = 0; i < haystack.length() - needle.length() + 1; i++) {
int j;
for (j = 0; j < needle.length(); j++) {
if (haystack[i + j] != needle[j]) break;
}
if (j == needle.length()) return i;
}
return -1;
}
};
- 时间复杂度: O(mn)
- 空间复杂度: O(1)