Recognizing a Pattern in My Recursive Functions

Shawn Drost from Hack Reactor taught me to write recursive functions with an if-else statement:

function recursion() {
  if(baseCase) {
    // do something
  } else {
    // get me 1 step closer to the base case
  }
}

As I was reviewing some of my curriculum material from Hack Reactor and thrashing about at codewars, I started to recognize a common pattern in my code when dealing with permutation problems that built on top of Shawn’s suggestion.

Here’s some code I would write if I had to find all the different permutations of a string called problem.

function findAllAnswers(problem) {
  var partialAnswer = "";
  var allAnswers = [];

  findAnswers(problem);

  return allAnswers;

  function findAnswer(problem) {
    if(problem.length === 0) {
      allAnswers.push(partialAnswer);
      return;
    }
    else {
      // add the first bit of 'problem' to partialAnswer
      partialAnswer.push(problem[0]);
      // explore all branches that include this first bit
      findAnswer(problem.slice(1));
      // lets remove what we added just before the recursive call
      partialAnswer = partialAnswer.substring(0, partialAnswer.length - 1);
    }
  }
}

I used this approach to solve the N-Queens problem, list all possibilities of a rock-paper-scissors matchup, and find all permutations of words you could be typing into a T-9 cell phone numpad.

The key components that jump out to me are:

1. I only have one partialAnswers variable that I manipulate until it meets the criteria for being a complete answer. Then I push the single solution to my allAnswers array.
2. I explore all branches of the first possibility, then backtrack one step at a time until I return to my original state. At that point, I start exploring all branches for the next possibility.
3. My code ends up being mostly simple and clean. I only need to worry about one parameter/argument in this simple problem. (My previous, unrefined approach would have required two arguments: remainingProblem and answerSoFar, which can be a bit of a mess to keep in order.)

The sorcery that happens in the else statement is something I may have previously struggled to come up with, but now it feels like child’s play. (That’s good, right?)

Next up? Maybe tail recursion.