151. Reverse Words in a String

1. 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.

2. Example

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.

Example 4

Input: s = " Bob Loves Alice "
Output: “Alice Loves Bob”

Example 5

Input: s = “Alice does not even like bob”
Output: “bob like even not does Alice”

3. Note

• 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.

4. Solutions

Two Pointers

n = s.size()
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    string reverseWords(string words) {
        reverse(words.begin(), words.end());

        int i = 0, j = 0, n = words.size();
        while (j < n) {
            while (j < n && words[j] == ' ') {
                ++j;
            }

            int left = i;
            while (j < n && words[j] != ' ') {
                words[i++] = words[j++];
            }
            reverse(words.begin() + left, words.begin() + i);

            if (i > left) {
                words[i++] = ' ';
            }
        }

        return words.substr(0, i - 1);
    }
};