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;
}
}