15. 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] ]

 

public List<List<Integer>> threeSum(int[] nums) {
    if (nums == null || nums.length == 0) {
        return new ArrayList<>();
    }
    Arrays.sort(nums);
    List<List<Integer>> res = new LinkedList<>();
    for (int i = 0; i < nums.length - 2; i++) {
        if (i == 0 || nums[i] != nums[i - 1]) {
            int lo = i + 1;
            int hi = nums.length - 1;
            int sum = -nums[i];
            while (lo < hi) {
                if (nums[lo] + nums[hi] == sum) {
                    res.add(Arrays.asList(nums[i], nums[lo], nums[hi]));
                    while (lo < hi && nums[lo] == nums[lo + 1]) //略过重复的元素
                        lo++;
                    while (lo < hi && nums[hi] == nums[hi - 1])
                        hi--;
                    lo++;
                    hi--;
                } else if (nums[lo] + nums[hi] < sum) {
                    lo++;
                } else {
                    hi--;
                }
            }
        }
    }
    return res;
}