day 26: 268-missing-number, 15-3-sum

15-3-sum/driver.cpp

@@ -0,0 +1,18 @@
+#include "soln.hpp"
+
+int main()
+{
+	vector<int> nums{-2,0,1,1,2};
+	Solution soln;
+	vector<vector<int>> result = soln.threeSum(nums);
+	cout << "Three sum triplets:" << endl;
+	for (auto itr : result)
+	{
+		for (int num : itr)
+		{
+			cout << num << " ";
+		}
+		cout << endl;
+	}
+	return 0;
+}

15-3-sum/soln.cpp

@@ -0,0 +1,47 @@
+#include "soln.hpp"
+
+vector<vector<int>> Solution::threeSum(vector<int> &nums)
+{
+	vector<vector<int>> result;
+	sort(nums.begin(), nums.end());
+	int low, high, sum;
+	vector<int> new_triplet;
+	for (int i = 0; i < nums.size() - 2; i++)
+	{
+		if (i > 0 && nums[i] == nums[i - 1])
+			continue;
+		// two sum
+		int low = i + 1, high = nums.size() - 1;
+		while (low < high)
+		{
+			// if(nums[low] == nums[low-1])    {
+			//     low++;
+			//     continue;
+			// }
+			// if(nums[high] == nums[high+1])    {
+			//     high--;
+			//     continue;
+			// }
+			sum = nums[i] + nums[low] + nums[high];
+			if (sum == 0)
+			{
+				new_triplet = vector<int>{nums[i], nums[low], nums[high]};
+				// if(find(result.begin(), result.end(), new_triplet) == result.end()) {
+				result.push_back({nums[i], nums[low], nums[high]});
+				// }
+				low++;
+				while (nums[low] == nums[low - 1] && low < high)
+					low++;
+			}
+			else if (sum > 0)
+			{
+				high--;
+			}
+			else
+			{
+				low++;
+			}
+		}
+	}
+	return result;
+}

15-3-sum/soln.hpp

@@ -0,0 +1,7 @@
+#include <bits/stdc++.h>
+using namespace std;
+class Solution
+{
+public:
+	vector<vector<int>> threeSum(vector<int> &nums);
+};

268-missing-number/driver.cpp

@@ -0,0 +1,10 @@
+#include "soln.hpp"
+
+int main()
+{
+	vector<int> nums{9, 6, 4, 2, 3, 5, 7, 0, 1};
+	int answer = 8;
+	Solution soln;
+	cout << "Found missing number correctly? " << soln.test(nums, answer) << endl;
+	return 0;
+}

268-missing-number/soln.cpp

@@ -0,0 +1,18 @@
+#include "soln.hpp"
+
+int Solution::missingNumber(vector<int> &nums)
+{
+	int n = nums.size();
+	int calc_sum = (n * (n + 1)) / 2;
+	int act_sum = 0;
+	for (int num : nums)
+	{
+		act_sum += num;
+	}
+	return calc_sum - act_sum;
+}
+
+bool Solution::test(vector<int> &nums, int answer)
+{
+	return missingNumber(nums) == answer;
+}

268-missing-number/soln.hpp

@@ -0,0 +1,8 @@
+#include <bits/stdc++.h>
+using namespace std;
+class Solution
+{
+public:
+	int missingNumber(vector<int> &nums);
+	bool test(vector<int> &nums, int answer);
+};

thoughts.md

@@ -4,3 +4,4 @@
   - essentials: bits/stdc++.h, iostream
   - data structures: vector
 - (q21)when working with structs, can't return locally created objects from a function, even when using new keyword...?
+- 2sum, 3sum, palindrome, and other two pointer problems can prolly be identified by how they require comparison-based solutions across the entire array, and the array is to be sorted or ordered in some way for traversal