【Leetcode】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> &num) {
        vector<vector<int>> result;
        if (num.size() < 3) return result;
        sort(num.begin(), num.end());
        for (size_t i = 0; i < num.size() - 2; ++i) {
            if (i > 0 && num[i] == num[i - 1]) continue;
            size_t j = i + 1, k = num.size() - 1;
            while (j < k) {
                if (j > i + 1 && num[j] == num[j - 1]) {
                    ++j;
                    continue;
                }
                if (k < num.size() - 1 && num[k] == num[k + 1]) {
                    --k;
                    continue;
                }
                int sum = num[i] + num[j] + num[k];
                if (sum == 0) {
                    result.push_back({num[i], num[j], num[k]});
                    ++j;
                    --k;
                } else if (sum < 0) {
                    ++j;
                } else {
                    --k;
                }
            }
        }
        return result;
    }
};

先排序，再左右夹逼查找。输出要求不重复，所以需要进行一些剪枝处理。