leetcode - 4Sum

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:
      std::vector
      
        > fourSum(std::vector
       
         &num, int target) { std::vector
        
         > resultSet; int n = num.size(); if (n < 4) { return resultSet; } sort(num.begin(), num.end()); std::vector
         
           result(4); for (int dIndex = n - 1; dIndex >= 3; dIndex--) { if ((dIndex < (n - 1)) && (num[dIndex + 1] == num[dIndex])) { continue; } result[3] = num[dIndex]; for (int cIndex = dIndex - 1; cIndex >= 2; cIndex--) { if ((cIndex < (dIndex - 1)) && (num[cIndex + 1] == num[cIndex])) { continue; } int threeSum = target - num[dIndex] - num[cIndex]; result[2] = num[cIndex]; int start = 0; int end = cIndex - 1; while (start < end) { int curSum = num[start] + num[end]; if (curSum == threeSum) { result[0] = num[start]; result[1] = num[end]; resultSet.push_back(result); do { start++; end--; } while (start < end && num[start] == num[start - 1] && num[end] == num[end + 1]); } else if (curSum > threeSum) { end--; } else { start++; } } } } return resultSet; } };