The sum of triplets are not always 0 C# LeetCode

[awkwineh183]

I have a hard time understanding the algorithm behind this LeetCode puzzle

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It gives me the following “wrong” answers:

My current code is as follows. I think I was supposed to tweak the conditions under my for-loops (like I need to watch out how big the last number on the array is etc). Here my head started spinning. Any hints much appreciated:

public class Solution {
    public IList<IList<int>> ThreeSum(int[] nums) {
        var res = new List<IList<int>>();
        Array.Sort(nums); 
        
        for(var i = 0; i < nums.Length; i++){
            for(var j = 0; j < nums.Length; j++){
                for(var k = 0; k < nums.Length; k++){
                    
                    if(j != k && i != j && i != k){
                        res.Add(new List<int>(){nums[i], nums[j], nums[k]});
                    }
                }
            }
        }
       return res;
    }
    
    
  
}

[midlindner]

Hello, @awkwineh183. Sorry for the slow reply. Seeing your post, I decided to attempt this myself. It proved to be much more challenging than I anticipated. Looking at what you have so far, your code returns a list of every possible combination of 3 values that have unique indices. You only want those that have a sum of 0. You also can’t have duplicates in your returned list. [-1, 0, 1] and [0, 1, -1] are considered duplicates in this challenge. You could alter your code a bit, and only add lists of numbers that have a sum of 0, but you need to come up with a way to avoid returning duplicates. Then you’ll need to work on optimizing the code in order to pass all of the tests in the time allotted. Good luck!

[awkwineh183]

Thank you so much for your advice – I think my code is almost close to the right solution:

public class Solution {
    public IList<IList<int>> ThreeSum(int[] nums) {
        var res = new List<IList<int>>();
        Array.Sort(nums); 
        
        for(var i = 0; i < nums.Length; i++){
            for(var j = 0; j < nums.Length; j++){
                for(var k = 0; k < nums.Length; k++){
                    
                    if((nums[j] != nums[k] && nums[i] != nums[j] && nums[i] != nums[k]) && (nums[i] + nums[j] + nums[k] == 0)){
                        res.Add(new List<int>(){nums[i], nums[j], nums[k]});
                    }
                }
            }
        }
       return res;
    }
    
    
  
}

Still it gives me errors, but I see some improvements in it (I got lesser wrong outputs than I did it first time).

I think I need to do something about this line if((nums[j] != nums[k] && nums[i] != nums[j] && nums[i] != nums[k]) && (nums[i] + nums[j] + nums[k] == 0)){ This is more like a dirty code I guess.

Maybe do you have any suggestion/links so I can get some hints to clean up this code?

[midlindner]

j, k, and i should not be equal, but the ints in nums at those indices can be. [0, 0, 0] and [-1, -1, 2] would be acceptable results if the input were { -1, -1, 0, 0, 0, 2 } for example.

[awkwineh183]

Ok - my code is almost there, but I can’t understand the error:

public class Solution {
    public IList<IList<int>> ThreeSum(int[] nums) {
        var res = new List<IList<int>>();
        Array.Sort(nums); 
        
        for(var i = 0; i < nums.Length-2; i++){
            for(var j = i+1; j < nums.Length-1; j++){
                for(var k = j+1; k < nums.Length; k++){
                    
                    if(nums[i] + nums[j] + nums[k] == 0){
                        res.Add(new List<int>(){nums[i], nums[j], nums[k]});
                    }
                }
            }
        }
       return res;
    }
    
    
  
}

And the error is;

my output: [-1, 0, 1] [-1, 2, -1] [-1,-1,2]

Could you kindly tell me what I did wrong here again?

[midlindner]

@awkwineh183

my output: [-1, 0, 1] [-1, 2, -1] [-1,-1,2]

You have to keep duplicates out of your result list. They consider a list with the same three integers in any order to be duplicates, so [-1, 2, -1] and [-1, -1, 2] are duplicates.