Array.push正在推送一个空数组，而不是我传递给它的完整数组

Question

I'm writing a solution for the Climbing Staircase Algorithm.
The prompt is specifically:

'You need to climb a staircase that has n steps, and you decide to get some extra exercise by jumping up the steps. You can cover at most k steps in a single jump. Return all the possible sequences of jumps that you could take to climb the staircase, sorted.'

My code works - I utilized backtracking, and it should all come out in the correct order EXCEPT... whenever I push a solution into my answer array, an empty array is getting pushed instead. I console logged to see what was getting pushed into my array, and it's a full array with the correct values. I cannot understand how this is happening. Here is my code:

function climbingStaircase(n, k) {
  const solutions = [];

  const findSolution = (progressArray, k, remaining) => {
      if (remaining === 0) {
          console.log('pA:', progressArray)
          solutions.push(progressArray);
          return;
      } else {
          for (let i = 1; i <= k; i++){
             if (i <= remaining) {
                progressArray.push(i);
                findSolution(progressArray, k, remaining - i);
                progressArray.pop();
              }
          }
      }
  }
  findSolution([], k, n);
  console.log('final solutions', solutions)
}

With the inputs n = 4 and k = 2, this is what my console looks like:

pA: [ 1, 1, 1, 1 ]
pA: [ 1, 1, 2 ]
pA: [ 1, 2, 1 ]
pA: [ 2, 1, 1 ]
pA: [ 2, 2 ]
final solutions [ [], [], [], [], [] ]

I've tried assigning interim variables to ensure that it's always referencing the correct array. I've tried including my solutions array as a parameter that I pass in... all of which seemed unnecessary, and still returned the same results. I know it's something small that is going to make me roll my eyes, but I cannot see it. Please help.

Answer 1

Your code is using one array throughout, and popping all of the entries it pushes into that array. You probably want to save a copy of the array when saving to solutions :

solutions.push(progressArray.slice());
// -------------------------^^^^^^^^

Live Example:

 function climbingStaircase(n, k) { const solutions = []; const findSolution = (progressArray, k, remaining) => { if (remaining === 0) { console.log('pA:', progressArray) solutions.push(progressArray.slice()); return; } else { for (let i = 1; i <= k; i++){ if (i <= remaining) { progressArray.push(i); findSolution(progressArray, k, remaining - i); progressArray.pop(); } } } } findSolution([], k, n); console.log('final solutions', solutions) } climbingStaircase(4, 2); 

 .as-console-wrapper { max-height: 100% !important; } 

Answer 2

Replace solutions.push(progressArray); by solutions.push(progressArray.slice()); .

You're passing the progressArray into your function, then you're mutating it. But since that's always the same reference to progressArray , you're mutating the same array. Eventually, progressArray.pop(); will remove all elements and you end up with five times the same empty array.

.slice() creates a copy of your array.