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[leetcode] 3Sum

3Sum

Given an array S of n integers, are there elements abc 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)

思路:

和3Sum Closet差不多,只是这次存的是集合。并且如果找到三个数之和等于0的时候,要注意第二个数或者第三个数要变化一下,否则就会陷入死循环中。

题解:

技术分享
class Solution {public:    vector<vector<int> > threeSum(vector<int> &num) {        vector<vector<int> > res;        vector<int> tmp;        sort(num.begin(), num.end());        for(int i=0;i<num.size();i++) {            while(i>0 && num[i]==num[i-1])                i++;            int l = i+1;            int r = num.size()-1;            while(l<r) {                while(l>i+1 && num[l]==num[l-1])                    l++;                while(r<num.size()-1 && num[r]==num[r+1])                    r--;                if(l<r) {                    int sum = num[i]+num[l]+num[r];                    if(sum>0)                        r--;                    else if(sum<0)                        l++;                    else {                        tmp.clear();                        tmp.push_back(num[i]);                        tmp.push_back(num[l]);                        tmp.push_back(num[r]);                        res.push_back(tmp);                        l++;                    }                }            }        }        return res;    }};
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[leetcode] 3Sum