[leecode]Implement strStr()

Implement strStr()

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

实现String.class中的contains()接口。

KMP算法

不了解该算法的，请移步KMP算法小结。这个也属于必须要掌握的算法。

【未通过case】needle == “”

代码如下：

public class Solution {
    public String strStr(String haystack, String needle) {
        if(haystack == null || needle == null || haystack.length() < needle.length() || (haystack.length() == needle.length() && !haystack.equals(needle)) ) return null;//needle为空串要特别注意
        if(needle.equals("") || haystack.equals(needle)) return haystack;
        int i = 0, j = 0;
        int[] next = new int[needle.length()];
        setNext(needle,next);
        while(i < haystack.length()){
            if(j == -1 || haystack.charAt(i) == needle.charAt(j)){
                i++;
                j++;
            }else{
                j = next[j];
            }
            if(j == needle.length()) return haystack.substring(i - needle.length());
        }
        return null;
    }
    private void setNext(String needle,int[] next){
        char[] p = needle.toCharArray();
        int length = p.length;
        int k = -1, j = 0;
        next[0] = -1;
        while(j < length - 1){
            if(k == -1 || p[k] == p[j]){
                next[++j] = ++k;
            }else{
                k = next[k];
            }
        }
    }

}

这道题我整理了整整一天，第一次手动的实现了KMP，并作出KMP算法小结。后天还要再复习一下。