leetcode-15

題目：

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:

• Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
• 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)

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代碼：

public class Solution {
      public List<List<Integer>> threeSum(int[] nums){
    int left, right;
    int len = nums.length;
    List<List<Integer>> res = new ArrayList<List<Integer>> ();
    if(nums == null || nums.length < 3)
    return res;
    Arrays.sort(nums);
   
    for(int i = 0; i < len - 2; i++){
     
    if(i != 0 && nums[i] == nums[i - 1] )
    continue;
   
    left = i + 1;
    right = len - 1;
    while(left < right)
    {
    if(nums[i] + nums[left] + nums[right] == 0)
    {
    List<Integer> list = new ArrayList<>();
    list.add(nums[i]);
    list.add(nums[left]);
    list.add(nums[right]);
    res.add(list);
    left++;
    right--;
    while(left < right && nums[left] == nums[left-1])
    left++;
    while(left < right && nums[right] == nums[right+1])
    right--;
    }
    else if(nums[i] + nums[left] + nums[right] < 0)
    left++;
    else
    right--;
    }
    }
    return res;
   
    }
}