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Method 1: The conventional approach is to intercept and compare the images one at a time.
class Solution {
public int strStr(String haystack, String needle) {
int haylen = haystack.length();
int neelen = needle.length();
if(haylen < neelen) return -1;
for(int i = 0; i <haylen-neelen+1;i++){
if(needle.equals(haystack.substring(i,i+neelen))) return i;
}
return -1;
}
}
Method 2: KMP matching algorithm
Mainly divided into two steps
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Step 1: Construct the next prefix array based on the matching string (version without minus 1)
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Step 2: Match the text string according to the constructed next array.
class Solution {
public int strStr(String haystack, String needle) {
int[] next = getNext(needle);
int j = 0;//前缀匹配过程
for(int i = 0 ;i<haystack.length();i++){
//注意这里是while 不是if while是需要一直找下去 直到找到相同前缀
while(j>0&&haystack.charAt(i) != needle.charAt(j)) j = next[j-1];
if(haystack.charAt(i) == needle.charAt(j)) j++;
if(j == next.length) return i - next.length +1;
}
return -1;
}
//构造next前缀表数组
public int[] getNext(String s){
int[] next = new int[s.length()];
int j = 0;
next[0] = j;//初始化 0 号位置的相同前后缀 肯定就是0
for(int i = 1 ; i<s.length() ; i++){
//注意这里是while 不是if while是需要一直找下去 直到找到相同前缀
while(j>0 &&s.charAt(i) != s.charAt(j)){
j = next[j-1];//一直找下去 直到找到相同前后缀 否则就是直接归0
}
if(s.charAt(i) == s.charAt(j)){
//若相等 前缀末尾j++ 然后再赋给前缀末尾i位置的next数组
j++;
}
next[i] = j;
}
return next;
}
}