Interview Question : Trim multiple consecutive spaces from a string

Question

This is an interview question Looking for best optimal solution to trim multiple spaces from a string. This operation should be in-place operation.

input  =          


        

Answer 1

I'd propose a little state machine (just a simple switch statement). Because if the interviewer is anything like me, the first enhancement they'll ask you to do is to fully trim any leading or trailing spaces, so that:

"    leading and    trailing    "

gets transformed to:

"leading and trailing"

instead of:

" leading and trailing "

This is a really simple modification to a state-machine design, and to me it seems easier to understand the state-machine logic in general over a 'straight-forward' coded loop, even if it takes a few more lines of code than a straight-forward loop.

And if you argue that the modifications to the straight forward loop wouldn't be too bad (which can be reasonably argued), then I (as the interviewer) would throw in that I also want leading zeros from numbers to be be trimmed.

On the other hand, a lot of interviewers might actually dislike a state-machine solution as being 'non-optimal'. I guess it depends on what you're trying to optimize.

Answer 2

Keep two indices: The next available spot to put a letter in (say, i), and the current index you're examining (say, j).

Just loop over all the characters with j, and whenever you see a letter, copy it to index i, then increment i. If you see a space that was not preceded by a space, also copy the space.

I think this would work in-place...

Answer 3

Here it is using only stdio:

#include <stdio.h>

int main(void){
    char str[] = "I    Like    StackOverflow a      lot";
    int i, j = 0, lastSpace = 0;
    for(i = 0;str[i]; i++){
        if(!lastSpace || str[i] != ' '){
            str[j] = str[i];
            j++;
        }
        lastSpace = (str[i] == ' ');
    }
    str[j] = 0;
    puts(str);
    return 0;
}

Answer 4

void trimspaces(char * str){
    int i = 0;
    while(str[i]!='\0'){
        if(str[i]==' '){
            for(int j = i + 1; j<strlen(str);j++){
                if(str[j]!=' '){
                    memmove(str + i + 1, str + j, strlen(str)-j+1);
                    break;
                }
            }
        }
        i++;
    }
}

Answer 5

Functional variant in Haskell:

import Data.List (intercalate)

trimSpaces :: String -> String
trimSpaces =  intercalate " " . words

The algorithm the next:

1. breaks a string up into a list of words, which were delimited by white space
2. concatenate the list inserting one space between each element in list