15. 3Sum

15. 3Sum

题目

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: 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]
]

解析

• 想写出一次能AC的代码真不容易！
• 很多细节问题，和sumtwo不一样的是：这次有重复元素；sumtwo假定没有重复元素，且只需要返回下标值
• 要跳过重复的元素

class Solution_15 {
public:

	void twosum(vector<vector<int>>& vecs,vector<int>& nums,int start, int target)
	{
		vector<int> ans;
		int end = nums.size() - 1;
		while (start<end)
		{
			if (nums[start]+nums[end]==target)
			{
				ans.push_back(-target);
				ans.push_back(nums[start]);
				ans.push_back(nums[end]);
				vecs.push_back(ans);
				ans.clear();
				//start++; end--;  跳不过重复的元素
				while (start<end&&nums[start]==nums[start+1])
				{
					start++;
				}
				while (start<end&&nums[end]==nums[end-1])
				{
					end--;
				}
				start++; end--;
			}else if (nums[start] + nums[end] < target)
			{
				start++;
			}
			else
			{
				end--;
			}
		}
		return;
	}

	vector<vector<int>> threeSum(vector<int>& nums) { // Time Limit Exceeded
		vector<vector<int>> vecs;
		if (nums.size() <= 2)
		{
			return vecs;
		}
		sort(nums.begin(), nums.end());
		for (int i = 0; i < nums.size() - 2;++i)
		{
			if (i>0&&nums[i]==nums[i-1]) //忽略掉有重复元素的值
			{
				continue;
			}
			twosum(vecs, nums,i+1, -nums[i]);
		}
		return vecs; //
	}

	vector<vector<int>> threeSum1(vector<int>& nums) { // Time Limit Exceeded
		vector<vector<int>> vecs;
		if (nums.size()<=2)
		{
			return vecs;
		}

		unordered_map<int, int> mp;
		vector<int> ans;
		for (int i = 0; i < nums.size() - 2;++i)
		{
			mp.clear();
			for (int j = i + 1; j < nums.size();++j)
			{
				auto iter = mp.find(-(nums[i] + nums[j])); //查找主关键字
				if (iter!=mp.end())
				{
					ans.push_back(nums[i]);
					ans.push_back(iter->first);
					ans.push_back(nums[j]);
					sort(ans.begin(), ans.end());  //处理有重复元素
					if (find(vecs.begin(),vecs.end(),ans)==vecs.end()) //没有元素才插入操作
					{
						vecs.push_back(ans);
					}
					ans.clear();
				}
				else
				{
					mp.insert(make_pair(nums[j], j));
				}
			}
		}
		return vecs; //
	}

};

题目来源

• 15. 3Sum