标签：you www effective lan change index int ack lang
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().
标签：you www effective lan change index int ack lang
原文地址：https://www.cnblogs.com/ruruozhenhao/p/9748607.html
