leetcode-28 Implement strStr()

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问题描述：

Implement strStr().

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

Update (2014-11-02):
The signature of the function had been updated to returnthe index instead of the pointer. If you still see your functionsignature returns a char * or String, please click the reload button  toreset your code definition.

问题分析：KMP算法，Java里面字符串要注意判断null以及是否为空字符串；

                优化的nextval算法见：http://blog.csdn.net/woliuyunyicai/article/details/44622573

代码：

public class Solution {
    public int strStr(String haystack, String needle) {
		if(haystack == null || needle == null)
			return -1;
		//注意初始化条件
		int i = -1;
		int j = -1;
		int[] next = get_next(needle);	
		
		while(i < haystack.length() && j < needle.length())
		{
			if(j == -1 || haystack.charAt(i) == needle.charAt(j))
			{
				++i;++j;
			}
			else
			{
				j = next[j];
			}
		}
		//判断是否匹配成功
		if(j >= needle.length())
			return i - j;
		return -1;
	}

	//next[j]计算函数
	private int[] get_next(String needle)
	{
		if(needle.length() == 0)
			return null;
		int[] next = new int[needle.length()];
		//初始化
		next[0] = -1;
		int i = 0;
		int j = -1;
		
		while(i < needle.length() - 1)
		{
			if(j == -1 || needle.charAt(i) == needle.charAt(j))
				next[++i] = ++j;
			else
				j = next[j];
		}	
        return next;		
	}
}

leetcode-28 Implement strStr()

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原文地址：http://blog.csdn.net/woliuyunyicai/article/details/45476649