Question

Given an array of characters which forms a sentence of words, give an efficient algorithm to reverse the order of the words (not characters) in it.

Example input and output:

>>> reverse_words('this is a string') 'string a is this' 

It should be O(N) time and O(1) space (split() and pushing on / popping off the stack are not allowed).

The puzzle is taken from here.

Answer 1

A solution in C/C++:

void swap(char* str, int i, int j){     char t = str[i];     str[i] = str[j];     str[j] = t; }  void reverse_string(char* str, int length){     for(int i=0; i<length/2; i++){         swap(str, i, length-i-1);     } } void reverse_words(char* str){     int l = strlen(str);     //Reverse string     reverse_string(str,strlen(str));     int p=0;     //Find word boundaries and reverse word by word     for(int i=0; i<l; i++){         if(str[i] == ' '){             reverse_string(&str[p], i-p);             p=i+1;         }     }     //Finally reverse the last word.     reverse_string(&str[p], l-p); } 

This should be O(n) in time and O(1) in space.

Edit: Cleaned it up a bit.

The first pass over the string is obviously O(n/2) = O(n). The second pass is O(n + combined length of all words / 2) = O(n + n/2) = O(n), which makes this an O(n) algorithm.