Implement strStr()

Description

For a given source string and a target string, you should output the first index(from 0) of target string in source string.

If target does not exist in source, just return -1.Have you met this question in a real interview? Yes

Clarification

Do I need to implement KMP Algorithm in a real interview?

• Not necessary. When you meet this problem in a real interview, the interviewer may just want to test your basic implementation ability. But make sure you confirm with the interviewer first.

Example

If source = "source" and target = "target", return -1.

If source = "abcdabcdefg" and target = "bcd", return 1.

Challenge

O(n2) is acceptable. Can you implement an O(n) algorithm? (hint: KMP)

这道题让我们在一个字符串中找另一个字符串第一次出现的位置，那我们首先要做一些判断，如果子字符串为空，则返回0，如果子字符串长度大于母字符串长度，则返回-1。然后我们开始遍历母字符串，我们并不需要遍历整个母字符串，而是遍历到剩下的长度和子字符串相等的位置即可，这样可以提高运算效率。然后对于每一个字符，我们都遍历一遍子字符串，一个一个字符的对应比较，

public class Solution {
    /**
     * @param source: 
     * @param target: 
     * @return: return the index
     */
    public int strStr(String source, String target) {
        // Write your code here
        if(target == null || source == null) return -1;
        
        if(target.length() == 0){
            return 0;
        }
        
        
        
        if( source.length() < target.length()){
            return -1;
        }
        
        for (int i = 0; i <= source.length() - target.length();i++ ){
            int j = 0;
            for(j = 0; j < target.length();j++ ){
                if(target.charAt(j) != source.charAt(i+j)){
                    break;
                }
            }
            
            if(j == target.length()){
                return i;
            }
        } 
        
        return -1;
        
        
    }
}