题解 | #二叉树的中序遍历#

1、双指针

import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        if (num == null || num.length == 0) {
            return res;
        }
        int n = num.length;
        Arrays.sort(num);
        for (int i = 0; i < n; i++){
            if (i > 0 && num[i] == num[i - 1]) continue;
            int l = i + 1, r = n - 1;
            int x = -num[i];
            while (l < r) {
                if (num[l] + num[r] < x) l++;
                else if (num[l] + num[r] > x) r--;
                else {
                    ArrayList<Integer> list = new ArrayList<>();
                    list.add(num[i]);
                    list.add(num[l]);
                    list.add(num[r]);
                    res.add(list);
                    while (l < r && num[l] == num[++l]);
                    while (l < r && num[r] == num[--r]);
                }
            }
        }
        return res;
    }
}