递归

public class 递归 {
	class TreeNode{
		int val;
		TreeNode left;
		TreeNode right;
		public TreeNode(int value){
			this.val=value;
		}
	}
	public int getHeight(TreeNode root){
		if(root==null){
			return 0;
		}
		int leftheight=getHeight(root.left);
		int rightheight=getHeight(root.right);
		return Math.max(leftheight, rightheight)+1;
	}

}

非递归

1.后序遍历二叉树

思路：按后序遍历二叉树，节点最大栈长即为二叉树的高度。

public class Solution {
    public int TreeDepth(TreeNode root) {
		if(root==null){
			return 0;
		}
		int height=0;
		Stack<TreeNode> nodes=new Stack<>();
		Stack<Integer> tag=new Stack<>();
		while(root!=null||!nodes.isEmpty()){
			while(root!=null){
				nodes.push(root);
				tag.push(0);
				root=root.left;
			}
			if(tag.peek()==1){
				height=Math.max(height, nodes.size());
				nodes.pop();
                tag.pop();
				root=null;
			}else{
				root=nodes.peek();
				root=root.right;
				tag.pop();
				tag.push(1);
			}
		}
		return height;
	}
}

2.按层次遍历二叉树

思路：按层次遍历二叉树，使用队列！！！！

public class 层次遍历二叉树求高度 {
	static class TreeNode{
		int val;
		TreeNode left;
		TreeNode right;
		public TreeNode(int value){
			this.val=value;
		}
	}
	public static int getHeight(TreeNode root){
		if(root==null){
			return 0;
		}
		Queue<TreeNode> queue=new LinkedList<>();
		queue.add(root);
		int height=1;
		while(!queue.isEmpty()){
			TreeNode node=queue.peek();
			if(node.left==null&&node.right==null){
				break;
			}else{
				if(node.left!=null){
					queue.add(node.left);
				}
				if(node.right!=null){
					queue.add(node.right);
				}
				queue.poll();
				height++;
			}
			
			//System.out.println(height);
			
		}
		return height;
	}
	
	public static void main(String[] args){
		TreeNode root=new TreeNode(1);
		root.left=new TreeNode(2);
		root.right=new TreeNode(3);
		System.out.println(getHeight(root));
	}

}