3Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:

[
  [-1, 0, 1],
  [-1, -1, 2]
]

Tips:

O(n^2) 先sort,注意重复数字一定要排除掉！

    if (i > 0 && nums[i] == nums[i - 1]) continue;
    while (left < right && nums[left + 1] == nums[left]) left++;
    while (left < right && nums[right - 1] == nums[right]) right--;

Code:

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