3Sum.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

class Solution {
  public List<List<Integer>> threeSum(int[] nums) {
    Arrays.sort(nums);
    List<List<Integer>> res = new ArrayList<>();
    for (int i = 0; i < nums.length-2; i++) {
      if (i == 0 || (i > 0 && nums[i] != nums[i - 1])) {
        int low = i + 1, high = nums.length - 1, target = -nums[i];
        while (low < high) {
          if (nums[low] + nums[high] == target) {
            res.add(Arrays.asList(nums[i], nums[low], nums[high]));
            while (low < high && nums[low] == nums[low + 1])
              low++;
            while (low < high && nums[high] == nums[high - 1])
              high--;
            low++;
            high--;
          } else if (nums[low] + nums[high] < target) {
            low++;
          } else {
            high--;
          }
        }
      }
    }
    return res;
  }
}