二叉树前序非递归遍历

package com.basic.bt;

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

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

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

    public void preOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        //访问跟 -> 左孩子 -> 右孩子
        result.add(root.val);
        preOrder(root.left);
        preOrder(root.right);
    }

    // no recursion
    public void preOrderTraversal(TreeNode root)
    {
        if(root == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode node = stack.pop();
            pre.add(node.val);
            if(node.right != null) {
                stack.push(node.right);
            }
            if(node.left != null) {
                stack.push(node.left);
            }
        }

    }

    public  static void main(String[] args) {
        PreOrderBT pre = new PreOrderBT();
        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;
        pre.preOrderTraversal(root);
        for(int i = 0; i < pre.pre.size(); i++) {
            System.out.println(pre.pre.get(i));
        }
    }

}