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3Sum

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Question

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]
]

Solution

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/*
Algorithm: First, get two sum by 0 - nums[index];
then, similarly solve two sum problem;
*/
public List<List<Integer>> threeSum(int[] nums) {
    List<List<Integer>> result = new ArrayList<>();
    if (nums.length < 3) return result;
    
    //[-1, 0, 1, 2, -1, -4]
    Arrays.sort(nums);
    for (int i = 0; i < nums.length - 2; i++) {
        if (i > 0 && nums[i] == nums[i-1]) {
            continue;
        }
        int target = 0 - nums[i];        
        int left = i + 1, right = nums.length - 1;            
        CalculateTwoSum(nums, target, left, right, result);
    }
    return result;
}

public void CalculateTwoSum(int[] nums,
                            int target,
                            int left, 
                            int right, 
                            List<List<Integer>> result) {        
    while (left < right) {            
        if (nums[left] + nums[right] == target) {
            List<Integer> candidate = new ArrayList<>();
            candidate.add(-target);
            candidate.add(nums[left]);
            candidate.add(nums[right]);                                        
            result.add(candidate);
            left++;
            right--;
            while (left < right && nums[left] == nums[left-1]) {
                left++;
            }
            while (left < right && nums[right] == nums[right+1]) {
                right--;
            }
        }else if (nums[left] + nums[right] < target) {
            left++;
        }else {
            right--;
        }
    }
}