[leetcode] 3Sum

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)

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：

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;
    }
};