二叉树后序非递归遍历

Posted 2020-08-28 notesbuddy

package com.basic.bt;

import java.util.ArrayList;
import java.util.Stack;

/**
 * Created by mac on 2017/1/19.
 */
public class PostOrderBT {

    ArrayList<Integer> result = new ArrayList<Integer>();

    public void postorder(TreeNode root) {
        if(root == null) {
            return;
        }
        postorder(root.left);
        postorder(root.right);
        result.add(root.val);
    }

    public void postOrderTraversal(TreeNode root) {

        if(root == null) {
            return;
        }

        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode node = root;
        TreeNode pre = null;

        while( node != null || !stack.isEmpty()) {
            while(node != null) {
                stack.push(node);
                node = node.left;
            }

            // node指向了空，所以要pop()一下  另外也是出于 对node右孩子的考虑;
            node = stack.pop();
            while( node != null && (node.right == null || node.right == pre)) {
                result.add(node.val);
                pre = node;
                if(stack.isEmpty()) {
                    return;
                }
                node = stack.pop();
            }

            stack.push(node);
            node = node.right;
        }

    }


    public  static void main(String[] args) {
        PostOrderBT in = new PostOrderBT();
        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;

        in.postOrderTraversal(root);

        for(int i = 0; i < in.result.size(); i++) {
            System.out.println(in.result.get(i));
        }

    }

}