统计二叉树结点个数

package com.basic.bt;

import java.util.Stack;

/**
 * 思路: 所谓计算个数，实际上是把每个结点遍历一遍
 * （1）递归
 * （2）非递归
 */
public class CountNodes {

    int count = 0;
    //inorder
    public void countNodes(TreeNode root) {
        if(root == null) {
            return;
        }
        countNodes(root.left);
        count++;
        countNodes(root.right);
    }


    public void countPreOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        //当最后一个节点出栈时，就是遍历结束的时候，所有while循环里面不用判断 root是否为空，且root指针从未改变过。 中序遍历时需要主要
        while(!stack.isEmpty()) {
            TreeNode tmp = stack.pop();
            count++;
            if(tmp.right != null) {
                stack.push(tmp.right);
            }
            if(tmp.left != null) {
                stack.push(tmp.left);
            }
        }
    }


    //
    public void countInOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while(root != null || !stack.isEmpty()) {

            while(root != null) {
                stack.push(root);
                root = root.left;
            }
            TreeNode node = stack.peek();
            stack.pop();
            count++;
            root = node.right;
        }
    }

    public void countPostOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode pre = null;
        TreeNode node = root;
        while(node != null || !stack.isEmpty()) {
            if(node != null) {
                stack.push(node);
                node = node.left;
            }
            node = stack.pop();
            count++;
            while(node!= null && (node.right == null || node.right == pre)) {
                count++;
                pre = node;
                //先判断是否为空，再pop   当只有最后一个节点的时候，本场景可以思考成功
                if(stack.isEmpty()) {
                    return;
                }
                node = stack.pop();
            }
            count++;
            stack.push(node);
            node = node.right;
        }

    }

    public static void main(String[] args) {
        CountNodes test = new CountNodes();
        TreeNode root = new TreeNode(0);
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);
        TreeNode node4 = new TreeNode(4);
        root.left = node1;
        root.right = node2;
        node1.left = node3;
        node1.right =node4;
        test.countPostOrder(root);
        System.out.println("二叉树的结点个数为：" + test.count);
    }
}