add: 220420 [cpp]

cpp/2204/220420.cpp

@@ -0,0 +1,60 @@
+#include <stack>
+
+struct TreeNode {
+	int val;
+	TreeNode* left;
+	TreeNode* right;
+
+	explicit TreeNode(int x = 0, TreeNode* l = nullptr, TreeNode* r = nullptr) : val(x), left(l), right(r) {}
+};
+
+/**
+ * 173. Binary Search Tree Iterator
+ * Implement the BSTIterator class that represents an iterator over the in-order traversal of a binary search tree (BST):
+ * BSTIterator(TreeNode root) Initializes an object of the BSTIterator class. The root of the BST is given as part of the constructor. The pointer should be initialized to a non-existent number smaller than any element in the BST.
+ * boolean hasNext() Returns true if there exists a number in the traversal to the right of the pointer, otherwise returns false.
+ * int next() Moves the pointer to the right, then returns the number at the pointer.
+ * Notice that by initializing the pointer to a non-existent smallest number, the first call to next() will return the smallest element in the BST.
+ * You may assume that next() calls will always be valid. That is, there will be at least a next number in the in-order traversal when next() is called.
+ */
+
+class BSTIterator {
+private:
+	std::stack<const TreeNode*> s;
+
+public:
+	explicit BSTIterator(const TreeNode* root) {
+		for (; root; root = root->left)
+			s.push(root);
+	}
+
+	int next() {
+		if (s.empty())
+			return -1;
+		const TreeNode* ret = s.top();
+		s.pop();
+		if (ret->right)
+			for (const TreeNode* ptr = ret->right; ptr; ptr = ptr->left)
+				s.push(ptr);
+		return ret->val;
+	}
+
+	bool hasNext() {
+		return !s.empty();
+	}
+};
+
+int main() {
+	int x;
+	bool b;
+	BSTIterator bSTIterator = BSTIterator(new TreeNode(7, new TreeNode(3), new TreeNode(15, new TreeNode(9), new TreeNode(20))));
+	x = bSTIterator.next();    // return 3
+	x = bSTIterator.next();    // return 7
+	b = bSTIterator.hasNext(); // return True
+	x = bSTIterator.next();    // return 9
+	b = bSTIterator.hasNext(); // return True
+	x = bSTIterator.next();    // return 15
+	b = bSTIterator.hasNext(); // return True
+	x = bSTIterator.next();    // return 20
+	b = bSTIterator.hasNext(); // return False
+}

cpp/2204/CMakeLists.txt

@@ -3,4 +3,4 @@ PROJECT(2204)
 
 SET(CMAKE_CXX_STANDARD 23)
 
-ADD_EXECUTABLE(2204 220419.cpp)
+ADD_EXECUTABLE(2204 220420.cpp)