151. Reverse Words in a String

151. Reverse Words in a String

1 Description #

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

Given an input string s, reverse the order of the words.

A word is defined as a sequence of non-space characters. The words in s will be separated by at least one space.

Return a string of the words in reverse order concatenated by a single space.

Note that s may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.

Example 1:

Input: s = "the sky is blue"
Output: "blue is sky the"

Example 2:

Input: s = "  hello world  "
Output: "world hello"
Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3:

Input: s = "a good   example"
Output: "example good a"
Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

Constraints:

1 <= s.length <= 10^4
s contains English letters (upper-case and lower-case), digits, and spaces ' '.
There is at least one word in s.

Follow-up: If the string data type is mutable in your language, can you solve it in-place with O(1) extra space?

2 Solution #

#include <sstream>
class Solution {
public:
  string reverseWords(string s) {
    // Time complexity: O(N), N is the size of s
    // Space complexity: O(N)
    int last_index = s.size() - 1;

    std::string result;
    std::string word;
    while(last_index >= 0){
      if(s[last_index] == ' '){
	if(!word.empty()){
	  result = result.empty() ? word : result + " " + word;
	}
	word = "";
      }else{
	word = s[last_index] + word;
      }
      last_index--;
    }

    if(!word.empty()){
      result = result.empty() ? word : result + " " + word;
    }

    return result;
  }
};