[LeetCode]3Sum

题目描述:(链接)

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)

解题思路:

先对数组进行排序，然后遍历数组，左右夹逼。

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> result;
        if (nums.size() < 3) { return result; }
        sort(nums.begin(), nums.end());
        
        auto last = nums.end();
        for (auto i = nums.begin(); i != last - 2; ++i) {
            if (i > nums.begin() && *i == *(i - 1)) { continue; }
            auto j = i + 1;
            auto k = last - 1;
            
            while (j < k) {
                int sum = *i + *j + *k;
                if (sum < 0) {
                    ++j;
                    while (*j == *(j - 1) && j < k) { ++j; }
                } else if (sum > 0) {
                    --k;
                    while (*k == *(k + 1) && j < k) { --k; }
                } else {
                    result.push_back({*i, *j, *k});
                    ++j;
                    --k;
                    while (*j == *(j - 1) && *k == *(k + 1) && j < k) { ++j; }
                }
            }

        }
        
        return result;
    }
};