From a065570f5fa316b44c9495d828cdee63bff5f6f9 Mon Sep 17 00:00:00 2001
From: "Md. Nazmul Hossain" <nazmulwanted@gmail.com>
Date: Mon, 11 Jul 2022 23:01:02 +0600
Subject: [PATCH 1/3] Problem 1: Maximum depth of a binary tree. Done.

---
 week09/maximum_depth_of_a_binary_tree.cpp | 7 +++++++
 1 file changed, 7 insertions(+)
 create mode 100644 week09/maximum_depth_of_a_binary_tree.cpp

diff --git a/week09/maximum_depth_of_a_binary_tree.cpp b/week09/maximum_depth_of_a_binary_tree.cpp
new file mode 100644
index 00000000..cee96f9e
--- /dev/null
+++ b/week09/maximum_depth_of_a_binary_tree.cpp
@@ -0,0 +1,7 @@
+// Time complexity: O(N), N = Number of nodes.
+// Space complexity: O(1)
+
+int maxDepth(TreeNode* root, int depth = 0){
+    if(!root) return depth;
+    return max(maxDepth(root->left, depth + 1), maxDepth(root->right, depth + 1));
+}

From 123edccc88ebe043677671693e7d22939dc7d75a Mon Sep 17 00:00:00 2001
From: "Md. Nazmul Hossain" <nazmulwanted@gmail.com>
Date: Wed, 13 Jul 2022 04:18:41 +0600
Subject: [PATCH 2/3] Problem 4: Number of islands. Done.

---
 week09/number_of_islands.cpp | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)
 create mode 100644 week09/number_of_islands.cpp

diff --git a/week09/number_of_islands.cpp b/week09/number_of_islands.cpp
new file mode 100644
index 00000000..f844b6c7
--- /dev/null
+++ b/week09/number_of_islands.cpp
@@ -0,0 +1,31 @@
+// Time complexity: O(M * N), M = Number of rows, N = Number of columns of the grid.
+// Space complexity: O(1)
+
+int dr[4] = {-1, 0, 1, 0};
+int dc[4] = {0, 1, 0, -1};
+
+void dfs(vector<vector<char>> &grid, int row, int col){
+    int r = grid.size();
+    int c = grid[0].size();
+    if(row == r || col == c || row < 0 || col < 0) return;
+    if(grid[row][col] == '0' || grid[row][col] == '2') return;
+    grid[row][col] = '2';
+    for(int i = 0; i < 4; i++){
+        dfs(grid, row + dr[i], col + dc[i]);
+    }
+}
+
+int numIslands(vector<vector<char>> &grid){
+    int row = grid.size();
+    int col = grid[0].size();
+    int count = 0;
+    for(int i = 0; i < row; i++){
+        for(int j = 0; j < col; j++){
+            if(grid[i][j] == '1'){
+                count++;
+                dfs(grid, i, j);
+            }
+        }
+    }
+    return count;
+}

From bf09d43cc7951efc3d281439aacbd686e1cf0ac3 Mon Sep 17 00:00:00 2001
From: "Md. Nazmul Hossain" <nazmulwanted@gmail.com>
Date: Thu, 21 Jul 2022 00:17:56 +0600
Subject: [PATCH 3/3] Problem 6: Course schedule. Done.

---
 week09/course_schedule.cpp | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)
 create mode 100644 week09/course_schedule.cpp

diff --git a/week09/course_schedule.cpp b/week09/course_schedule.cpp
new file mode 100644
index 00000000..edb22b0c
--- /dev/null
+++ b/week09/course_schedule.cpp
@@ -0,0 +1,27 @@
+// Time complexity: O(N), N = Number of courses.
+// Space complexity: O(N), N = Number of courses.
+
+bool hasCycle(vector<int> &col, vector<vector<int>> &graph, int u){
+    col[u] = 1;
+    bool status = false;
+    for(auto v: graph[u]){
+        if(col[v] == 1) status = true;
+        else if(col[v] == 0){
+            status |= hasCycle(col, graph, v);
+        }
+    }
+    col[u] = 2;
+    return status;
+}
+
+bool canFinish(int numCourses, vector<vector<int>> &prerequisites){
+    vector<vector<int>> graph(numCourses);
+    vector<int> col(numCourses, 0);
+    for(auto pre: prerequisites){
+        graph[pre[1]].push_back(pre[0]);
+    }
+    for(int i = 0; i < numCourses; i++){
+        if(hasCycle(col, graph, i)) return false;
+    }
+    return true;
+}