LeetCode·15. 3Sum

題目：

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]
]

思路：

2-sum問題基礎上加一層循環

代碼：

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> vec;
        if(nums.size() == 0)return vec;

        sort(nums.begin(), nums.end());

        vector<int> v(3);
        int sum = 0;
        for(int i = 0; i < nums.size()-2; ++i){
            if(nums[i] > 0) return vec;
            sum = 0 - nums[i];
            v[0] = nums[i];
            for(int j = i+1, k = nums.size()-1; j <= k-1;){
                if(nums[j]+nums[k] == sum){
                    v[1] = nums[j];
                    v[2] = nums[k];
                    vec.push_back(v);
                    if(nums[j] != nums[k]){
                        while((j < k) && nums[++j] == nums[j-1]){}
                        while((j < k) && nums[--k] == nums[k+1]){}
                    }else{
                        break;
                    }
                }else if(nums[j]+nums[k] < sum){
                    ++j;
                }else if(nums[j]+nums[k] > sum){
                    --k;
                }
            }

            while(i < nums.size()-2 && nums[i+1] == nums[i])
                ++i;
        }

        return vec;
    }
};

結果：