28. Implement strStr()

28. Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function(haystack, needle) {
    //题目是想求needle是不是haystack是子串。是的话返回开始匹配的index，否者就返回-1
    
    //暴力的写法就是全部都试过一边，不过kmp是比较好的实现方式
    //kmp算法最难的是next表的建立
    
    function next(pattern){
        
        var next = [];        
        var len = pattern.length; 
        next[0] = -1;
        
        var k = -1;
        var j = 0;
        
        while(j < len-1){
            
            //此刻，k即next[j-1]，且pattern[k]表示前缀，pattern[j]表示后缀
            //注:k==-1表示未找到k前缀和k后缀相等，首次分析可先忽略
            
            if(k == -1 || pattern[j] == pattern[k]){            
                ++j;
                ++k;
                next[j] = k;
            }else{  //pattern[j] 与 pattern[k]不相等，则继续递归计算前缀p[next[k]]
                
                k = next[k];
            }
            
        }
        return next;
    }
    
    var next = next(needle);
    
    var i = 0,j = 0;
    
    while(i < haystack.length && j < needle.length){
        
       //当j=-1时，要移动的是i，当然j也要归0
        if(j == -1 || haystack[i] == needle[j]){
            
            i++;
            j++;
            
        }else{
            
            //i不需要回溯了
            //i = i - j +  1;
            
            j = next[j]; //j回到指定位置
        
        }
        
    }
    
    if(j == needle.length){
        
        return i - j;
        
    }else{
        
        return -1;
    }
    
};