3Sum 題解

原創文章，拒絕轉載

題目來源：https://leetcode.com/problems/3sum/description/

Description

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.

Example

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

Solution

class Solution {
private:
    vector<vector<int> > twoSum(vector<int>& nums, int end, int target) {
        vector<vector<int> > res;
        int low = 0
 
;
        int high = end;
        while (low < high) {
            if (nums[low] + nums[high] == target) {
                vector<int> sum(2);
                sum[0] = nums[low++];
                sum[1] = nums[high--];
                res.push_back(sum);

                // 去重
                while
 
 (low < high && nums[low] == nums[low - 1])
                    low++;
                while (low < high && nums[high] == nums[high + 1])
                    high--;
            } else if (nums[low] + nums[high] > target) {
                high--;
            } else {
                low++;
            }
        }
        return res;
    }
public:
    vector<vector<int> > threeSum(vector<int>& nums) {
        vector<vector<int>> res;
        int size = nums.size();
        if (size < 3)
            return res;
        sort(nums.begin(), nums.end());
        for (int i = size - 1; i >= 2; i--) {
            if (i < size - 1 && nums[i] == nums[i + 1])  // 去重
                continue;
            auto sum2 = twoSum(nums, i - 1, 0 - nums[i]);
            if (!sum2.empty()) {
                for (auto& sum : sum2) {
                    sum.push_back(nums[i]);
                    res.push_back(sum);
                }
            }
        }
        return res;
    }
};

解題描述

這道題是Two Sum的進階，解法上采用的是先求Two Sum再根據求到的sum再求三個數和為0的第三個數，不過題意要求不一樣，Two Sum要求返回數組下標，這道題要求返回具體的數組元素。而如果使用與Two Sum相同的哈希法去做會比較麻煩。

這裏求符合要求的2數之和用的方法是，先將數組排序之後再進行夾逼的辦法。並且為了去重，需要在2sum和3sum都進行去重。

[Leetcode] 3Sum