三个数之和

public ArrayList> threeSum(int[] num) {
    ArrayList> resultList = new ArrayList<>();
    int len = num.length;
    if (num == null || len < 3) {
        return list;
    }
    Arrays.sort(num);
    for (int i = 0; i < len; i++) {
        if (num[i] > 0) {
            return list;
        }
        if (i > 0 && num[i] == num[i - 1]) {
            continue;
        }
        int L = i + 1;
        int R = len - 1;
        while (L < R) {
            int sum = num[i] + num[L] + num[R];
            if (sum == 0) {
                ArrayList arrayList = new ArrayList<>();
                arrayList.add(num[i]);
                arrayList.add(num[L]);
                arrayList.add(num[R]);
                resultList.add(arrayList);
                while (L < R && num[L] == num[L + 1]) {
                    L++;
                }
                while (L < R && num[R] == num[R - 1]) {
                    R--;
                }
                L++;
                R--;
            } else if (sum < 0) {
                L++;
            } else if (sum > 0) {
                R--;
            }
        }
    }
    return resultList;
}