实现strStr()

package LeetCodeDemo.primary;

public class Demo38 {

    public static void main(String[] args){
        System.out.println(new Demo38().strStr("mississippi", "issipi"));
    }

    public int strStr(String haystack, String needle) {
        if (needle == null || needle.equals("")){
            return 0;
        }
        if (needle.length() > haystack.length()){
            return -1;
        }

        int start = 0;
        int i = 0;
        int j = 0;
        char[] arr1 = haystack.toCharArray();
        char[] arr2 = needle.toCharArray();
        while (start < haystack.length() && j < needle.length() ){
            if ( i < haystack.length() && arr1[i] == arr2[j]){ //匹配则都都加一
                i++;
                j++;
            }else {
                j = 0;
                start++;  // 不匹配 的情况下 重新比较即可 起始位置加一重新比较
                i = start;
            }

        }

        if (j != arr2.length){ //不存在匹配串
            return -1;
        }
        return  i - j; // 存在匹配串直接这样就行
    }

}