4Sum
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:
Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target?
Find all unique quadruplets in the array which gives the sum of target.
Note:
Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a <= b <= c <= d)
The solution set must not contain duplicate quadruplets.
For example, given array S = {1 0 -1 0 -2 2}, and target = 0.
A solution set is:
(-1, 0, 0, 1)
(-2, -1, 1, 2)
(-2, 0, 0, 2)
class Solution { public: vector<vector<int> > fourSum(vector<int> &num, int target) { int N = num.size(); vector<vector<int> > res; if (N < 4) return res; sort(num.begin(), num.end()); for (int i = 0; i < N; ++i) { if (i > 0 && num[i] == num[i-1]) continue; // avoid duplicates for (int j = i+1; j < N; ++j) { if (j > i+1 && num[j] == num[j-1]) continue; // avoid duplicates int twosum = target - num[i] - num[j]; int l = j + 1, r = N - 1; while (l < r) { int sum = num[l] + num[r]; if (sum == twosum) { vector<int> quadruplet(4); quadruplet[0] = num[i]; quadruplet[1] = num[j]; quadruplet[2] = num[l]; quadruplet[3] = num[r]; res.push_back(quadruplet); while (l < r && num[l+1] == num[l]) l++; // avoid duplicates while (l < r && num[r-1] == num[r]) r--; // avoid duplicates l++; r--; } else if (sum < twosum) l++; else r--; } } } return res; } };