Reverse Words in a String

Description

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 <= 104
• 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?

Code

Time Complexity: $O(n)$, Space Complexity: $O(1)$

class Solution {
public:
    string reverseWords(string s) {
        int i = 0, j = 0, n = s.length();
        int wordCount = 0;
        while(j < n) {
            while(j < n && s[j] == ' ') j++;
            if(j == n) break;
 
            if(wordCount) s[i++] = ' ';
 
            int l = i;
            while(j < n && s[j] != ' ') {
                s[i++] = s[j++];
            }
            if(i > l) {
                reverse(s.begin() + l, s.begin() + i);
                wordCount++;
            }
        }
        s = s.substr(0, i);
        reverse(s.begin(), s.end());
        return s;
    }
};

Source

• Reverse Words in a String - LeetCode