Javascript coding exercise

[camelcamper]

I am doing this coding exercise on leetcode.com and am getting unexpected output
https://leetcode.com/problems/plus-one/description/

The function is supposed to receive an array of single numbers, but you treat the array as a full number and add 1.

Example 1:

Input: [1,2,3]
Output: [1,2,4]
Explanation: The array represents the integer 123.

Example 2:

Input: [4,3,2,1]
Output: [4,3,2,2]
Explanation: The array represents the integer 4321.

My code

var plusOne = function(digits) {
    let str = ''
    for(let num of digits) {
        str += num
    }
    let num = parseInt(str);
    num++;
    let strArr = [...num.toString()]
    let finArr = []
    let temp = 0;
    for(let item of strArr) {
        temp = parseInt(item)
        finArr.push(temp)
    }
    
    return finArr
};

Input: [6,1,4,5,3,9,0,1,9,5,1,8,6,7,0,5,5,4,3]
Output: [6,1,4,5,3,9,0,1,9,5,1,8,6,7,0,5,0,0,0]
Expected: [6,1,4,5,3,9,0,1,9,5,1,8,6,7,0,5,5,4,4]

In this test case the final 3 indexes are getting pushed as 0's and I cannot figure out why.
Any ideas?

[hbar1st]

I think you should do a different implementation. Add 1 to the digit in the highest index position if the number is less than 8. If 9 then change 9 to 0 and add one to the previous index number (if it is less than 8) etc. So basically treat it like someone doing a vertical addition calculation in math class.

[ArielLeslie]

I think that @hbar1st has pointed you to the intention of the challenge.

[camelcamper]

Yeah I was thinking of a solution like this but I suck at recursion and am not too keen to implement a bunch of for’s and if’s.
I’ll see how it goes thanks

[camelcamper]

Thanks for the info.

Dont have to use JS but I am primarily practicing the JS for job interviews.

[hbar1st]

The trick to recursion is to know when to stop. So always code the stop condition first. That is, if you are given an array with a single number, how would you add one to it? If you can write this code to work for 0 to 9 then you can solve the whole thing.

However i imagine just looping backwards through the array will solve this, so no recursion needed.

[ArielLeslie]

This problem isn’t a bad candidate for recursion, but if I were to solve it recursively it would look very much like the imperative solution discussed here.

[camelcamper]

Thanks for everyones input, the info from @camperextraordinaire about math operations on large Javascript numbers is really good to know.

@camperextraordinaire

In JavaScript, the highest number which you can reliably perform math operations on is 9,007,199,254,740,991

Any number higher than that like your 6,145,390,195,186,705,543 will not produce numbers you expect.

I ended up with this solution built from the advice of @hbar1st

var plusOne = function(digits) {
    if(digits[digits.length-1] < 9) {
        digits[digits.length-1] += 1;
        return digits;
    }else{
      let i = digits.length-1;
      while(digits[i] == 9) {
        digits[i] = 0;
        if(i == 0) {
          digits.unshift(1);
          return digits;
        }
        i--;
      }
      digits[i] += 1;
    }
  return digits;
};

Ends up being a lot easier than I thought with a while loop, and much better than my first implementation.

[camelcamper]

Yeah cheers rando, I need to get better at reading/writing shorthand and ES6 JS syntax.

I had to read the ternary statement in your return like 10 times before I understood it.

[coder23]

@camperextraordinaire came up with a recursive solution. Might be on similar terms you’re thinking.