LeetCode15. 3Sum（C++/Python）

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]
]

解题思路：先将数组排序，然后开始遍历，先确定一个数，遇到这个数与前面的数相同时，则跳过，然后用双指针法，在该数的右边找两个数使得这三个数相加为0；

C++

class Solution {
public:
    vector> threeSum(vector& nums) {
        sort(nums.begin(), nums.end());
        vector> result;
        for(int i = 0; i < nums.size(); ++ i){
            while(i != 0 && i < nums.size() && nums[i-1] == nums[i])
                ++ i;
            if(i == nums.size())
                break;
            int target = -nums[i], low = i+1, high = nums.size() - 1;
            while(low < high){
                int temp = nums[low] + nums[high];
                if(temp == target){
                    result.push_back({nums[i], nums[low], nums[high]});
                    while(low < high && nums[low] == (*result.rbegin())[1])
                        ++ low;
                    while(low < high && nums[high] == (*result.rbegin())[2])
                        -- high;
                }
                else if(temp > target)
                    -- high;
                else
                    ++ low;
            }
        }
        return result;
    }
};

Python

class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        result = []
        nums = sorted(nums)
        for i in range(len(nums)):
            if i and nums[i] == nums[i-1]:
                continue
            low, high = i + 1, len(nums) - 1
            while low < high:
                temp = nums[i] + nums[low] + nums[high]
                if temp == 0:
                    result.append([nums[i], nums[low], nums[high]]);
                    while low < high and nums[low] == result[-1][1]:
                        low += 1
                    while low < high  and nums[high] == result[-1][2]:
                        high -= 1
                elif temp > 0:
                    high -= 1
                else:
                    low += 1
        return result