104. Maximum Depth of Binary Tree

104/step1.cpp

@@ -0,0 +1,19 @@
+/*
+Solve Time : 2:56
+
+Time : O(V + E)
+Space : O(V)
+
+特に問題なく解いた、ひとまず一番シンプルにかける再帰を使用したが、queueやstackを使用したほうがよいはず。
+*/
+class Solution {
+public:
+  int maxDepth(TreeNode* root) {
+    if (root == nullptr) {
+      return 0;
+    }
+    const int left_depth = maxDepth(root->left);
+    const int right_depth = maxDepth(root->right);
+    return max(left_depth, right_depth) + 1;
+  }
+};

104/step2_1.cpp

@@ -0,0 +1,23 @@
+/*
+Time : O(V + E)
+Space : O(V)
+*/
+class Solution {
+public:
+  int maxDepth(TreeNode* root) {
+    stack<pair<TreeNode*, int>> nodes_and_depths;
+    nodes_and_depths.emplace(root, 0);
+    int max_depth = 0;
+    while (!nodes_and_depths.empty()) {
+      auto [node, depth] = nodes_and_depths.top();
+      nodes_and_depths.pop();
+      if (node == nullptr) {
+        continue;
+      }
+      max_depth = max(max_depth, depth + 1);
+      nodes_and_depths.emplace(node->left, depth + 1);
+      nodes_and_depths.emplace(node->right, depth + 1);
+    }
+    return max_depth;
+  }
+};

104/step3.cpp

@@ -0,0 +1,19 @@
+class Solution {
+public:
+  int maxDepth(TreeNode* root) {
+    queue<pair<TreeNode*, int>> nodes_and_depths;
+    nodes_and_depths.emplace(root, 0);
+    int max_depth = 0;
+    while (!nodes_and_depths.empty()) {
+      auto [node, depth] = nodes_and_depths.front();
+      nodes_and_depths.pop();
+      max_depth = depth;
+      if (!node) {
+        continue;
+      }
+      nodes_and_depths.emplace(node->left, depth + 1);
+      nodes_and_depths.emplace(node->right, depth + 1);
+    }
+    return max_depth;
+  }
+};