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

My Accepted Solution

n = i_str.size()
Time complexity: O(n)
Space complexity: O(n)

class Solution 
{
public:
    // string reverseWords(string s)
    string reverseWords(string &i_str) 
    {
        // remove duplicate blank
        istringstream stream(i_str);
        
        string result;
        for(string iter; stream >> iter; result += iter + " ");
            
        if(result.empty()) return result;
        
        result.pop_back(); // we delete the last invalid blank
        
        // twice reverse
        reverse(result.begin(), result.end());
        for(int left = 0, right = 0; left < result.size() && right >= 0; left = right + 1)
        {
            right = result.find(' ', left);
            reverse(result.begin() + left, result.begin() + (right >= 0 ? right : (int)result.size()));
        }

        return result;
    }
};

4.1 Stream

n = i_str.size()
Time complexity: O(n)
Space complexity: O(n)

class Solution 
{
public:
    // string reverseWords(string s)
    string reverseWords(string &i_str) 
    {
        // remove duplicate blank
        istringstream stream(i_str);
        
        string result;
        stream >> result;
            
        for(string inserter; stream >> inserter; )
        {
            result = inserter + string(" ") + result;
        }

        return result;
    }
};