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 s) {
        reverse(s.begin(), s.end());

        int i = 0;
        for (int left = 0, right = 0; right < s.size(); right++) {
            while (right < s.size() && s[right] == ' ') {
                ++right;
            }

            left = i;
            while (right < s.size() && s[right] != ' ') {
                s[i++] = s[right++];
            }

            reverse(s.begin() + left, s.begin() + i);
            if (i > left) {
                s[i++] = ' ';
            }
        }

        s.resize(i - 1);

        return s;
    }
};