https://leetcode.com/problems/reverse-words-in-a-string/

reverseWordsInAString.js

/**
 * @param {string} s
 * @return {string}
 */

// snazzy one liner
// o(n) time + o(n) space
var reverseWords = function(s) {
  return s.trim().replace(/\s\s+/g, ' ').split(' ').reverse().join(' ');
};

// theoretically o(1) space, o(n) time
var reverseWords = function(s) {
    const sen = [];
    
    let i = 0;
    for(i=0; i<s.length && s[i] === ' '; i++); // ignore starting spaces
    for(; i<s.length; i++){                    // but from the first real character
        if(s[i-1] === ' ' && s[i] === ' ') continue; // always ignoring consecutive spaces
        sen.push(s[i]);                              // push the entire sentence
    }
    while(sen.length && sen[sen.length-1] === ' ') sen.pop(); // pop trailing spaces
    
    sen.reverse(); // now reverse the entire sentence
    
    let [l, r] = [0, 0];          // then reverse individual words
    while(r < sen.length){
        if(sen[r] === ' '){       // if we arrive at a space
            reverse(l, r-1, sen); // reverse everything preceding
            l = r+1;              // and init the left boundary of the next word
        }
        r++;
    }
    
    reverse(l, sen.length-1, sen); // and reverse the last word
    
    return sen.join('');
};

function reverse(l, r, array){
    while(l < r){
        [array[l], array[r]] = [array[r], array[l]];
        l++; r--;
    }
}