15. 3Sum

Given an array nums of n integers, are there elements a, b, c in nums 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.Example:Given array nums = [-1, 0, 1, 2, -1, -4],A solution set is:[ [-1, 0, 1], [-1, -1, 2]]难度：medium题目：给定一长度为n的数组，是否存在三元素之和为0? 找出所有使其和为0的不同的三元组。注意：返回的结果中不要含有重复元素。思路：数组排序后使用双重循环固定一个值，再使内循环由两头向中间计算。Runtime: 56 ms, faster than 52.39% of Java online submissions for 3Sum.class Solution { public List<List<Integer>> threeSum(int[] nums) { List<List<Integer>> result = new ArrayList<>(); if (null == nums || nums.length < 1) { return result; } Arrays.sort(nums); int n = nums.length; for (int i = 0; i < n - 1; i++) { if (i > 0 && nums[i] == nums[i - 1]) { continue; } for (int left = i + 1, right = n - 1; left < right; ) { if (left > (i + 1) && nums[left] == nums[left - 1]) { left++; continue; } if (right < (n - 1) && nums[right] == nums[right + 1]) { right–; continue; } int sum = nums[i] + nums[left] + nums[right]; if (0 == sum) { result.add(Arrays.asList(nums[i], nums[left++], nums[right–])); } else if (sum > 0) { right–; } else { left++; } } } return result; }}