Add DFS implementation with README

DFS/README.md

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+# Depth-First Search (DFS)
+
+## What is DFS?
+
+**Depth-First Search (DFS)** is a graph and tree traversal algorithm that explores as far as possible along one path before backtracking and exploring another path.
+
+DFS can be implemented using:
+
+* **Recursion**
+* **Stack**
+
+## How DFS Works
+
+1. Start from a node.
+2. Mark the node as visited.
+3. Visit one of its unvisited neighbors.
+4. Continue going deeper until there are no unvisited neighbors.
+5. Backtrack and explore other paths.
+
+### Example
+
+For the graph:
+
+```text
+    A
+   / \
+  B   C
+ / \
+D   E
+```
+
+A possible DFS traversal is:
+
+```text
+A → B → D → E → C
+```
+
+## DFS in Word Search
+
+In the **Word Search** problem, DFS is used to explore neighboring cells to find the given word.
+
+From each cell, we can move in four directions:
+
+```text
+      Up
+       ↑
+Left ← Cell → Right
+       ↓
+     Down
+```
+
+A cell is temporarily marked as visited so that it is not used twice in the same path. After exploring the path, the cell is restored. This process is called **backtracking**.
+
+## Algorithm
+
+```text
+For every cell in the board:
+    Start DFS
+
+DFS(row, col, index):
+    If the complete word is found:
+        return true
+
+    If the cell is invalid or doesn't match:
+        return false
+
+    Mark the cell as visited
+
+    Search:
+        Up
+        Down
+        Left
+        Right
+
+    Restore the cell
+
+    Return whether any direction found the word
+```
+
+## Complexity
+
+For a graph with `V` vertices and `E` edges:
+
+* **Time:** `O(V + E)`
+* **Space:** `O(V)`
+
+For the Word Search problem, the worst-case time complexity is approximately:
+
+`O(m × n × 4^L)`
+
+where:
+
+* `m × n` = board size
+* `L` = length of the word
+
+## Applications
+
+DFS is commonly used for:
+
+* Graph and tree traversal
+* Finding connected components
+* Cycle detection
+* Maze solving
+* Path finding
+* Topological sorting
+* Backtracking problems
+* Word Search
+
+## Implementation
+
+The C++ implementation for Word Search is provided in `word_search.cpp`.

DFS/word_search.cpp

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+class Solution {
+public:
+    bool dfs(vector<vector<char>>& board, string word, int i, int j, int k) {
+        // All characters matched
+        if (k == word.size())
+            return true;
+
+        // Out of bounds or character doesn't match
+        if (i < 0 || i >= board.size() ||
+            j < 0 || j >= board[0].size() ||
+            board[i][j] != word[k])
+            return false;
+
+        // Mark as visited
+        char temp = board[i][j];
+        board[i][j] = '#';
+
+        // Explore 4 directions
+        bool found = dfs(board, word, i + 1, j, k + 1) ||
+                     dfs(board, word, i - 1, j, k + 1) ||
+                     dfs(board, word, i, j + 1, k + 1) ||
+                     dfs(board, word, i, j - 1, k + 1);
+
+        // Backtrack
+        board[i][j] = temp;
+
+        return found;
+    }
+
+    bool exist(vector<vector<char>>& board, string word) {
+        for (int i = 0; i < board.size(); i++) {
+            for (int j = 0; j < board[0].size(); j++) {
+                if (dfs(board, word, i, j, 0))
+                    return true;
+            }
+        }
+
+        return false;
+    }
+};