15. 3Sum

Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

The solution set must not contain duplicate triplets.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

先對nums排序，loop第一個數，再利用two pointers找第2/3個數，要記得跳過duplicate number。

vector<vector<int>> threeSum(vector<int>& nums) {
    sort(nums.begin(), nums.end());
    vector<vector<int> > res;
    int n = nums.size();
    for (int i = 0; i < n - 2; ++i) {
        if (i > 0 && nums[i] == nums[i - 1]) continue;
        int l = i + 1, r = n - 1;
        while (l < r) {
            int sum = nums[i] + nums[l] + nums[r];
            if (sum == 0) {
                res.push_back({nums[i], nums[l], nums[r]});
                while (l < r && nums[l] == nums[l + 1]) ++l;
                while (l < r && nums[r] == nums[r - 1]) --r;
                ++l, --r;
            } else if (sum < 0) {
                ++l;
            } else {
                --r;
            }
        }
    }
    return res;
}