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  • 3sum

    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:

    •Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
    •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)
    
    class Solution {
    public:
        vector<vector<int>> threeSum(vector<int>& nums) {
            sort(nums.begin(),nums.end());
            vector<vector<int>> ret;
            for(int i = 0;i < nums.size();i++)
            {
                int start =  i+1;
                int end = nums.size()-1;
                if(i>0&&nums[i]==nums[i-1])//关键点1:去除已经反复的点
                    continue;
                while(start<end)//转化为2sum问题
                {
    
                    while(start>i+1&&nums[start]==nums[start-1])//关键点2:排除反复的部分
                    {
                        start++;
                    }
                    while(end<nums.size()-1&&nums[end]==nums[end+1])
                    {
                        end--;
                    }
                    int sum = nums[start]+nums[i]+nums[end];
                    if(start<end)
                    {
                        if(sum == 0)
                        {
                            vector<int> temp;
                            temp.push_back(nums[i]);
                            temp.push_back(nums[start]);
                            temp.push_back(nums[end]);
                            ret.push_back(temp);
                            start++;
                        }else if(sum>0)
                        {
    
                            end--;
                        }else{
    
                            start++;
                        }
                    } 
                }
    
            }
            return ret;
        }
    };
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  • 原文地址:https://www.cnblogs.com/yangykaifa/p/7207492.html
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