Reverse words in a String

Author: AbdulWahab938

Arrays & Strings/#151- Reverse words in a String - Medium/Explanation.md

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+# 151. Reverse Words in a String
+
+**Difficulty:** *Easy*  
+**Category:** *Strings, Stack, Two Pointers*  
+**Leetcode Link:** [Problem Link](https://leetcode.com/problems/reverse-words-in-a-string/)
+
+---
+
+## 📝 Introduction
+
+Given a string `s`, reverse the order of words in it. A word is defined as a sequence of non-space characters. The words in `s` are separated by one or more spaces. The output string should only contain a single space separating the words, and no leading or trailing spaces.
+
+---
+
+## 💡 Approach & Key Insights
+
+The main idea is to reverse the order of the words, not the characters within them. We explore multiple approaches starting from brute force using a stack to a space-optimized method that constructs the result while traversing the string from the end.
+
+---
+
+## 🛠️ Breakdown of Approaches
+
+### 1️⃣ Brute Force / Naive Approach
+
+- **Explanation:**  
+  Push each word onto a stack while traversing the string. This reverses the order naturally. Then pop from the stack and build the output string. Stack ensures LIFO behavior to reverse the word order.
+  
+- **Time Complexity:** *O(N)* - We traverse the entire string once and then build the result string from the stack.  
+- **Space Complexity:** *O(N)* - Additional space for the stack and the answer string.
+
+- **Example/Dry Run:**
+
+```plaintext
+Input: "TUF is great for interview preparation"
+
+Step 1 → Push each word into a stack:
+Stack = ["TUF", "is", "great", "for", "interview", "preparation"]
+
+Step 2 → Pop and build answer:
+"preparation interview for great is TUF"
+
+Output: "preparation interview for great is TUF"
+```
+
+---
+
+### 2️⃣ Optimized Approach
+
+- **Explanation:**  
+  Traverse the string from the end, constructing each word in reverse. Append each complete word to the result as we detect spaces. This eliminates the need for a stack. We carefully manage whether a space is needed before appending each word.
+
+- **Time Complexity:** *O(N)* - Single traversal of the string.  
+- **Space Complexity:** *O(1)* - We only use a few extra variables (not counting the output string).
+
+- **Example/Dry Run:**
+
+```plaintext
+Input: "TUF is great for interview preparation"
+
+Step 1 → Start from end:
+"preparation" → add to result  
+"interview" → prepend to result  
+"for" → prepend  
+"great" → prepend  
+"is" → prepend  
+"TUF" → prepend
+
+Final Output: "preparation interview for great is TUF"
+```
+
+---
+
+### 3️⃣ Best / Final Optimized Approach (if applicable)
+
+- **Explanation:**  
+  The two-pointer approach traverses from end to start, building words and appending them to the result string efficiently. It avoids stack overhead and minimizes space usage.
+
+- **Time Complexity:** *O(N)*  
+- **Space Complexity:** *O(1)* (excluding result string)
+
+- **Example/Dry Run:**
+
+```plaintext
+Input: "  Hello   World  "
+
+Trim extra spaces  
+Traverse from the end  
+Build each word and append  
+Final Output: "World Hello"
+```
+
+---
+
+## 📊 Complexity Analysis
+
+| Approach      | Time Complexity | Space Complexity |
+| ------------- | --------------- | ---------------- |
+| Brute Force   | O(N)            | O(N)             |
+| Optimized     | O(N)            | O(1)             |
+| Best Approach | O(N)            | O(1)             |
+
+---
+
+## 📉 Optimization Ideas
+
+- Modify the input string in-place if mutable (e.g., using character arrays in C++).
+- Skip multiple spaces early using regex or manual trimming.
+
+---
+
+## 📌 Example Walkthroughs & Dry Runs
+
+```plaintext
+Example:
+Input: "TUF is great for interview preparation"
+Process:
+1. Extract words: ["TUF", "is", "great", "for", "interview", "preparation"]
+2. Reverse order: ["preparation", "interview", "for", "great", "is", "TUF"]
+3. Join with single spaces
+Output: "preparation interview for great is TUF"
+```
+
+---
+
+## 🔗 Additional Resources
+
+
+- [C++ String Functions](https://cplusplus.com/reference/string/string/)
+
+---
+
+Author: Abdul Wahab  
+Date: 19/07/2025

Arrays & Strings/Reverse Words in a String 3.java renamed to Arrays & Strings/#151- Reverse words in a String - Medium/Reverse Words in a String 3.java

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+class Solution {
+public:
+    string reverseWords(string s) {
+        int left = 0;
+        int right = s.length()-1;
+    
+        string temp="";
+        string ans="";
+    
+        //Iterate the string and keep on adding to form a word
+        //If empty space is encountered then add the current word to the result
+        while (left <= right) {
+            char ch= s[left];
+            if (ch != ' ') {
+                temp += ch;
+            } else if (ch == ' ') {
+                if (ans!="") ans = temp + " " + ans;
+                else ans = temp;
+                temp = "";
+            }
+            left++;
+        }
+    
+        //If not empty string then add to the result(Last word is added)
+        if (temp!="") {
+            if (ans!="") ans = temp + " " + ans;
+            else ans = temp;
+        }
+    
+        return ans; 
+    }
+};

Arrays & Strings/#151- Reverse words in a String - Medium/reverseWords.cpp

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+class Solution:
+    def reverseWords(self, s: str) -> str:
+        left = 0
+        right = len(s) - 1
+
+        temp = ""
+        ans = ""
+
+        # Skip leading spaces
+        while left <= right and s[left] == ' ':
+            left += 1
+        # Skip trailing spaces
+        while right >= left and s[right] == ' ':
+            right -= 1
+
+        # Iterate the string and collect words
+        while left <= right:
+            ch = s[left]
+            if ch != ' ':
+                temp += ch
+            else:
+                if temp != '':
+                    if ans != '':
+                        ans = temp + " " + ans
+                    else:
+                        ans = temp
+                    temp = ""
+            left += 1
+
+        # Add the last word if any
+        if temp != "":
+            if ans != "":
+                ans = temp + " " + ans
+            else:
+                ans = temp
+
+        return ans