Merge branch '7oSkaaa:main' into main

05- May/21- Shortest Bridge/21- Shortest Bridge (Lama Salah).cpp

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+// Author: Lama Salah
+
+/*
+
+-- THE IDEA --
+
+Use a combination of depth-first search (DFS) and breadth-first search (BFS) algorithms.
+The DFS function is used to mark the first island as visited and push its cells into the queue.
+The BFS function then performs a breadth-first search to find the shortest distance from the first island to the second island.
+
+*/
+
+class Solution {
+    int n; // Size of the grid.
+
+    // Direction arrays for exploring neighboring cells (right, left, down, up).
+    int dx[4] = {0, 0, 1, -1}; 
+    int dy[4] = {1, -1, 0, 0};
+
+    vector<vector<int>> grid; // Input grid
+    queue<vector<int>> q; // Queue for BFS
+
+public:
+    // 'valid' function to check if coordinates (x, y) are within the grid bounds.
+    bool valid(int x, int y) {
+        return x >= 0 && y >= 0 && x < n && y < n; 
+    }
+
+    void dfs(int i, int j) {
+        q.push({i, j}); // Push the current cell (i, j) into the queue.
+        grid[i][j] = -1; // Mark the current cell as visited (-1).
+
+        // Explore all four neighboring cells.
+        for (int k = 0; k < 4; k++) { 
+            // Calculate the new x-coordinate and y-coordinate of the neighboring cell.
+            int nx = i + dx[k];
+            int ny = j + dy[k]; 
+
+            // Check if the neighboring cell is within bounds and unvisited.
+            if (valid(nx, ny) && grid[nx][ny] == 1) { 
+                dfs(nx, ny); // Recursively call DFS on the neighboring cell.
+            }
+        }
+    }
+
+    int bfs(int i, int j) {
+        // Initialize the distance between the two islands as 0.
+        int distance = 0; 
+
+        // Process cells in the queue until it becomes empty.
+        while (!q.empty()) { 
+            int sz = q.size(); // Get the current number of cells in the queue.
+            while (sz--) {
+                // Get the front cell from the queue and pop it.
+                auto curr = q.front(); 
+                q.pop();
+
+                // Explore all four neighboring cells.
+                for (int k = 0; k < 4; k++) {
+                    // Calculate the new x-coordinate and y-coordinate of the neighboring cell.
+                    int nx = curr[0] + dx[k]; 
+                    int ny = curr[1] + dy[k]; 
+
+                    // Check if the neighboring cell is within bounds and unvisited.
+                    if (valid(nx, ny) && grid[nx][ny] != -1) { 
+                        // Check if the neighboring cell is the second island (the second island is found).
+                        // If it is, return the current distance.
+                        if (grid[nx][ny] == 1) { 
+                            return distance; 
+                        }
+
+                        // Mark the neighboring cell as visited (-1).
+                        grid[nx][ny] = -1; 
+                        // Push the neighboring cell into the queue.
+                        q.push({nx, ny}); 
+                    }
+                }
+            }
+
+            // Increment the distance after processing all cells in the current level.
+            distance++; 
+        }
+
+        // Return the shortest distance between the two islands.
+        return distance; 
+    }
+
+    int shortestBridge(vector<vector<int>>& g) {
+        this->grid = g; // Store the input grid.
+        this->n = grid.size(); // Get the size of the grid.
+
+        // Iterate through the grid to find the first island.
+        for (int i = 0; i < n; i++) { 
+            for (int j = 0; j < n; j++) {
+                // If a cell with value 1 is found (first island).
+                if (g[i][j] == 1) { 
+                    dfs(i, j); // Perform DFS to mark the first island as visited and push its cells into the queue.
+                    return bfs(i, j); // Call BFS to find the shortest bridge between the two islands.
+                }
+            }
+        }
+
+        return 0; // If no islands are found, return 0.
+    }
+};