19. Reverse Words

The problem can be found at the following link: Question Link

Problem Description

Given a string str, reverse the string without reversing its individual words. Words are separated by dots.

Note: The last character is not a dot.

Examples:

Input: str = "i.like.this.program.very.much" Output: "much.very.program.this.like.i"

Explanation: After reversing the whole string (not individual words), the input string becomes "much.very.program.this.like.i".

Input: str = "pqr.mno" Output: "mno.pqr"

Explanation: After reversing the whole string, the input string becomes "mno.pqr".

My Approach

1. Iterate Backwards:

   • Traverse the string from the last character to the first.

   • Every time a dot (.) is encountered, extract the word from the position after the dot to the current position and append it to the result string.

   • Continue this process until all words are appended in reverse order.

2. Final Touch:

   • Add the last word, which doesn't have a preceding dot, after the loop ends.

Time and Auxiliary Space Complexity

• Expected Time Complexity: O(|str|), where |str| is the length of the input string. This is because we are iterating over the string once to reverse the words.

• Expected Auxiliary Space Complexity: O(|str|), as we are using a new result string to store the reversed words.

Code (C++)

class Solution {
  public:
    string reverseWords(string str) {
        string result = "";
        int n = str.length();
        int end = n - 1;

        for (int i = n - 1; i >= 0; i--) {
            if (str[i] == '.') {
                result.append(str.substr(i + 1, end - i)).append(".");
                end = i - 1;
            }
        }
        result.append(str.substr(0, end + 1));

        return result;
    }
};

Code (Java)

class Solution {
    String reverseWords(String str) {
        StringBuilder result = new StringBuilder();
        int n = str.length();
        int end = n - 1;

        for (int i = n - 1; i >= 0; i--) {
            if (str.charAt(i) == '.') {
                result.append(str.substring(i + 1, end + 1)).append(".");
                end = i - 1;
            }
        }
        result.append(str.substring(0, end + 1));

        return result.toString();
    }
}

Code (Python)

class Solution:
    def reverseWords(self, str):
        result = []
        n = len(str)
        end = n - 1

        for i in range(n - 1, -1, -1):
            if str[i] == '.':
                result.append(str[i+1:end+1])
                result.append('.')
                end = i - 1
        result.append(str[:end+1])

        return ''.join(result)

Contribution and Support

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