leetcode : Longest Common Prefix : simple JavaScript solution

Posted on June 14, 2020

This is not the fastest solution’s by far, but I think that it is one of the more simple ones to understand.

var longestCommonPrefix = function(strs) {
    if(!strs[0]){return ""} //empty array case
    let lcp = "" //base case 
    //lcp can never be greater then first string 
    //so we can use it as the max length of our loop
    for(i=0;i<strs[0].length;i++){   
        let c=strs
            .map(s=>s[i])  //get all the i'th index letters 
            //get all the unique letters gathered in map
            .reduce((arr,char)=> arr.includes(char) ? arr : [...arr,char], [])
         if(c.length === 1){lcp += c[0]} //if unique letters == 1, keep looping
        else{break} //else loop is over
    }
    return lcp
};

leetcode : 867. Transpose Matrix

Posted on June 11, 2020

This one took a while to get. Originally I was trying to transpose in place, but you cannot guarantee the input is a square, so in place won’t work.Not sure if even in C you can transpose in place for a rectangular input. There maybe some memory tricks there, but they definitely are not available for JavaScript

Code is commented to guide

var transpose = function(A) {
    //this problem we actually cant transpose in place
    //since we cannot guarantee that the input is a square
    
    const result = []
    const rowsLength = A[0].length
    const columnsLength = A.length
    
    for(i=0;i<rowsLength;i++){
        let column = []
        for(j=0;j<columnsLength;j++){
            //here we get all elements in a column from input array A
            //and add it to new array named column
            column.push(A[j][i])
        }
        //we take the variable column we generated above
        //and turn it into a row for output Array result
        result.push(column)
    }
    return result
};

leetcode: Plus One : JavaScript submission beats 99.90% in memory usage

Posted on June 9, 2020 Updated on June 9, 2020

What the problem is asking for is to take the array and treat the whole thing as an integer. So [1,2,3] is the number 123, and if you add 1 that is 124. Then you would return [1,2,4].

You cannot just add 1 to the end, because for [5,6,9] you would need to return [5,7,0]. 569 + 1 = 570

Another thing to consider is that if the input is [9,9,9] then the returned array is larger having values [1,0,0,0]. 999 + 1 = 1000

The code I used is below. You can’t use the join, convert to number trick since some of the inputs are too large and then are rounded up loosing the trailing digits.

var plusOne = function(digits) {
    // return (parseInt(digits.join(""))+1).toString().split("")
    //the above looses precision when the number is too large
 
    for(let i=digits.length-1;i>=0; i--){
        if(digits[i]<9){digits[i]=digits[i]+1; return digits}
        digits[i]=0
    }
    //if we are here the first value in array was a 9
    digits.unshift(1)
    return digits
};

My runtime only beat 10.41% of the solutions. Great on memory, poor on performance.

leetcode: reverse integer: correct answer marked wrong

Posted on June 7, 2020 Updated on June 8, 2020

I was doing leetcode’s reverse integer which as it’s name implies expects an integer to be reverse. example: 562 should be returned as 265.

The website marked my solution wrong even though my solution returned a revered number. The testcase 1534236469 had returned 9646324351 by my solution. Leetcode expected 0

The reason this is happening is because leetcode expects to only deal with a “32-bit signed integer”, and the output is larger than the maximum value of a 32-bit signed integer.

The largest a 32 bit signed integer can be is 2,147,483,647. A bit smaller than 9,646,324,351. I adjusted my solution and now received an acceptance.

var reverse = function(x) {
    let isNegative = x < 0 || false
    let rArray = x.toString().split("")
    console.log("rArray is",rArray)
    if(isNegative){ rArray.shift()}
    let rInt = parseInt(rArray.reverse().join(""))
    if (rInt > 2147483647) {return 0}
    return (isNegative ? rInt - 2 * rInt : rInt)
};
//pseudocode
    //convert to string //split into array
    // if  negative is true shift first char
    //reverse array // convert to string //convert to num
    //if negative true, subtract 2 * x