strStr

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.

Example

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

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

看别人的博客 偶尔提到播放器算法 顺便写一写 复习一下

两个注意的地方

   一是输入检查 target 为空的情况 直接返回0;

   二是返回值计算的时候 是sIndex-tIndex 而不是 sIndex-tIndex+1 （第一次跑搞错了）

class Solution {
    /**
     * Returns a index to the first occurrence of target in source,
     * or -1  if target is not part of source.
     * @param source string to be scanned.
     * @param target string containing the sequence of characters to match.
     */
    int[] p;
    public int strStr(String source, String target) {
        // write your code here
        if(source==null||target==null||target.length()>source.length()) return -1;
        if(target.length()==0) return 0;
        preProcess(target);
        int sIndex =0, tIndex = 0;
        
        while(sIndex<source.length()){
            if(source.charAt(sIndex)==target.charAt(tIndex)){
                sIndex++;
                tIndex++;
            }else{
                if(tIndex>0){
                    tIndex = p[tIndex];
                }else{
                    sIndex++;
                }
            }
            if(tIndex==target.length()) return sIndex-tIndex;
        }
        return -1;
        
    }
    private void preProcess(String target){
        p = new int[target.length()];
        int i=1;
        while(i<target.length()){
            int j = p[i-1];
            while(j>0&&target.charAt(j)!=target.charAt(i)) j = p[j-1];
            if(target.charAt(j)==target.charAt(i)){
                p[i] = j+1;
            }
            i++;
        }
    }
}