二叉树相关算法常见笔试题

创建二叉树上一篇中已经提到：https://blog.csdn.net/weixin_39309402/article/details/101349676
1、如何计算二叉树叶子节点（没有子结点[即度为0]的结点称为叶子结点，简称“叶子”）的个数

// 叶子节点的个数(递归算法)
	public static int getNodeNumLeafRec(TreeNode root) {
		if (root == null)
			return 0;
		if (root.left == null && root.left == null)
			return 1;
		return getNodeNumLeafRec(root.left) + getNodeNumLeafRec(root.right);
	}

public static void main(String[] args) {
		TreeNode root = new TreeNode();
		root = createTree(root, 0);

		System.out.println("叶子节点的个数:" + new Test2().getNodeNumLeafRec(root)); // 3
		
	}

2、如何判断是否是二叉搜索树

/* 判断是否是二叉搜索树 */
	// 方法一：计算高度
	public int treedeep(TreeNode root) {
		if (root == null)
			return 0;
		int left = treedeep(root.left);
		int right = treedeep(root.right);
		return Math.max(left, right) + 1;
	}

	public boolean QueryTree(TreeNode root) {
		if (root == null)
			return true;
		int left = treedeep(root.left);
		int right = treedeep(root.right);
		if (Math.abs(right - left) > 1)
			return false;
		boolean treeleft = QueryTree(root.left);
		boolean treeright = QueryTree(root.right);
		return treeleft && treeright;
	}
	
	
	/**
	 * 验证二叉搜索树-方法二
	 */
	public boolean isvaild(TreeNode root) {
		if (root == null) {
			return true;
		}
		return valid(root, Long.MIN_VALUE, Long.MAX_VALUE);
	}

	private boolean valid(TreeNode root, long minValue, long maxValue) {
		if (root == null) {
			return true;
		}
		if (root.val <= minValue || root.val >= maxValue) {
			return false;
		}
		return valid(root.left, minValue, root.val) && valid(root.right, root.val, maxValue);
	}



public static void main(String[] args) {
		TreeNode root = new TreeNode();
		root = createTree(root, 0);

		System.out.println("判断是否是搜索二叉树:"+new Test2().QueryTree(root)); //false
		System.out.println("判断是否是搜索二叉树:"+new Test2().isvaild(root)); //false
	}

3、如何利用深度优先遍历二叉树？

// 深度优先遍历
	public static void preorderTraversal(TreeNode root) {
		if (root == null)
			return;
		Stack stack = new Stack<>();
		stack.push(root);
		while (!stack.isEmpty()) {
			TreeNode node = stack.pop();
			System.out.println(node.val);
			if (null != node.left) {
				stack.push(node.left);
			}
			if (null != node.right) {
				stack.push(node.right);
			}
		}

    }

	public static void main(String[] args) {
		TreeNode root = new TreeNode();
		root = createTree(root, 0);
		
		System.out.println("深度优先遍历二叉树:");
		new Test2().preorderTraversal(root); 
		
	}