二叉树的前中后非递归遍历

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

public class Solution {
	
	//前序非递归遍历，根 左 右
	public ArrayList<Integer> preOrderTraversal(TreeNode root){
		ArrayList<Integer> res = new ArrayList<Integer>();
		if(root==null){
			return res;
		}
		Stack<TreeNode> stack = new Stack<TreeNode>();
		stack.push(root);
		
		//对特定一个节点的访问顺序为，
		while(!stack.isEmpty()){
			TreeNode temp = stack.pop();
			res.add(temp.val);
			
			if(temp.right!=null){
				stack.push(temp.right);
			}
			if(temp.left!=null){
				stack.push(temp.left);
			}
		}
		return res;
	}
	
	//中序非递归遍历， 左 根 右
	public ArrayList<Integer> inOrderTraversal(TreeNode root){
		ArrayList<Integer> res = new ArrayList<Integer>();
		if(root==null){
			return res;
		}
		Stack<TreeNode> stack = new Stack<TreeNode>();
		
		while(root!=null || !stack.isEmpty()){
			while(root!=null){		//将当前节点，以及左子树一直入栈，循环结束时，root==null
				stack.push(root);
				root = root.left;
			}
			root = stack.pop();
			res.add(root.val);	//访问当前，左子树为空，或者已经被访问
			
			root = root.right;  //访问右子树
		}
		return res;
	}
	
	//后续非递归遍历二叉树，左 右 根
	public ArrayList<Integer> postOrderTraversal(TreeNode root){
		ArrayList<Integer> res = new ArrayList<Integer>();
		if(root==null){
			return res;
		}
		Stack<TreeNode> stack = new Stack<TreeNode>();
		stack.push(root);
		
		while(!stack.isEmpty()){
			TreeNode temp = stack.pop();
			res.add(0,temp.val);	
			if(temp.left!=null){	//先让左入栈，最后来访问
				stack.push(temp.left);
			}
			if(temp.right!=null){
				stack.push(temp.right);
			}
		}
		
		return res;
	}
	
	public static void main(String[] args) {
		Solution s = new Solution();
		
		TreeNode root = TreeNode.getTree();
		
		System.out.println(s.postOrderTraversal(root));
		
	}
}

参考地址：https://leetcode.com/discuss/100569/preorder-inorder-postorder-traversal-iterative-solution