题目大意：给你一个数组，让你找出3个数的和为0的所有组合。

Problem:

https://leetcode.com/problems/3sum/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.

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

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

Solution 1: Hashtable

Time complexity: O(n^2)
Space complexity: O(n)

// Author: Huahua
class Solution {
public:
  vector<vector<int>> threeSum(vector<int>& nums) { 
    sort(begin(nums), end(nums));
    const int n = nums.size();
    unordered_map<int, int> c;
    for (int x : nums) ++c[x];
    vector<vector<int>> ans;
    for (int i = 0; i < n; ++i) {
      if (i && nums[i] == nums[i - 1]) continue;
      for (int j = i + 1; j < n; ++j) {
        if (j - 1 != i && nums[j] == nums[j - 1]) continue;
        const int t = 0 - nums[i] - nums[j];
        // nums[i] <= nums[j] <= nums[k]
        if (t < nums[j]) continue; 
        if (!c.count(t)) continue;
        // Make sure we have enough count
        if (c[t] >= 1 + (nums[i] == t) + (nums[j] == t))
          ans.push_back({nums[i], nums[j], t});        
      }      
    }
    return ans;
  }
};

Solution 2: Sorting + Two pointers

Time complexity: O(nlogn + n^2)

Space complexity: O(1)

C++

class Solution {
public:
  vector<vector<int>> threeSum(vector<int>& nums) {
    vector<vector<int>> ans;
    std::sort(nums.begin(), nums.end());
    const int n = nums.size();
    for (int i = 0; i < n - 2; ++i) {
      if (nums[i] > 0) break;
      if (i > 0 && nums[i] == nums[i - 1]) continue;
      int l = i + 1;
      int r = n - 1;      
      while (l < r) {
        if (nums[i] + nums[l] + nums[r] == 0) {
          ans.push_back({nums[i], nums[l++], nums[r--]});
          while (l < r && nums[l] == nums[l - 1]) ++l;
          while (l < r && nums[r] == nums[r + 1]) --r;          
        } else if (nums[i] + nums[l] + nums[r] < 0) {
          ++l;
        } else {
          --r;
        }
      }
    }
    return ans;
  }
};

Related Problems:

• 花花酱 LeetCode 454. 4Sum II
• [解题报告] LeetCode 1. Two Sum 花花酱

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