LeetCode ---- 28、实现strStr()

题目链接

思路：使用KMP算法进行求解。

    public int strStr(String haystack, String needle) {
        if (needle.equals("")) {
            return 0;
        }
        char[] hayChar = haystack.toCharArray();
        char[] needleChar = needle.toCharArray();
        int[] next = getNextArray(needleChar);
        int i = 0;
        int j = 0;
        while (i < hayChar.length && j < needleChar.length) {
            if (hayChar[i] == needleChar[j]) {
                i++;
                j++;
            } else if (next[j] == -1) {
                i++;
            } else {
                j = next[j];
            }
        }
        return j == needleChar.length ? i - j : -1;
    }
    public int[] getNextArray(char[] arr) {
        if (arr.length == 1) {
            return new int[]{ -1 };
        }
        int[] next = new int[arr.length];
        next[0] = -1;
        next[1] = 0;
        int i = 2;
        int cn = 0;
        while (i < arr.length) {
            if (arr[i - 1] == arr[cn]) {
                next[i++] = ++cn;
            } else if (cn > 0) {
                cn = next[cn];
            } else {
                next[i++] = 0;
            }
        }
        return next;
    }

也可以使用内置优化过的indexOf方法来进行求解。

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