二叉樹遍歷的非遞迴演算法（java實現）

package com.mpackage.tree;
import java.util.*;

public class TreeSolution {
	
	//先根遍歷
	public static ArrayList<Integer> prePrint(TreeNode t){
		ArrayList<Integer> result=new ArrayList<Integer>();
		if(t==null){
			return result;
		}
		Stack<TreeNode> stack=
 
new Stack<TreeNode>();
		while(!stack.isEmpty()||t!=null){
			while(t!=null){
				result.add(t.val);
				stack.push(t);
				t=t.left;
			}
			if(!stack.isEmpty()){
				TreeNode tnode=stack.pop();
				t=tnode.right;
			}
		}
		return result;
	}
	
	//先序遍歷2
	public static ArrayList<Integer> prePrint2
 
(TreeNode t){
		ArrayList<Integer> list=new ArrayList<Integer>();
		if(t==null){
			return list;
		}
		Stack<TreeNode> stack=new Stack<TreeNode>();
		stack.push(t);
		TreeNode node;
		while(!stack.isEmpty()){
			node=stack.pop();
			list.add(node.val);
			if(node.right!=null)
 
{
				stack.push(node.right);
			}
			if(node.left!=null){
				stack.push(node.left);
			}
		}
		return list;
	}
	
	//中序遍歷
	public static ArrayList<Integer> midPrint(TreeNode t){
		ArrayList<Integer> result=new ArrayList<Integer>();
		if(t==null){
			return result;
		}
		Stack<TreeNode> stack=new Stack<TreeNode>();
		while(!stack.isEmpty()||t!=null){
			while(t!=null){
				stack.push(t);
				t=t.left;
			}
			if(!stack.isEmpty()){
				TreeNode tnode=stack.pop();
				result.add(tnode.val);
				t=tnode.right;
			}
		}
		return result;
	}
	
	//後序遍歷
	public static ArrayList<Integer> postPrint(TreeNode t){
		ArrayList<Integer> list=new ArrayList<Integer>();
		if(t==null){
			return list;
		}
		TreeNode node=null;
		TreeNode pre=null;//上一個訪問的節點
		Stack<TreeNode> stack=new Stack<TreeNode>();
		while(t!=null||!stack.isEmpty()){
			while(t!=null){
				stack.push(t);
				t=t.left;
			}
			if(!stack.isEmpty()){
				node=stack.peek();
				//如果該節點的右孩子為空或者等於上一個訪問的節點，則可以訪問該節點
				if(node.right!=null&&node.right!=pre){
					t=node.right;
				}
				else{
					stack.pop();
					list.add(node.val);
					pre=node;
				}
			}
		}
		return list;
	}
	
	//後序遍歷2
	//思路：後序遍歷順序為左->右->根,
	//可以按根->右->左遍歷,然後轉置;
	public static ArrayList<Integer> postPrint2(TreeNode t){
		ArrayList<Integer> list=new ArrayList<Integer>();
		if(t==null){
			return list;
		}
		Stack<TreeNode> stack=new Stack<TreeNode>();
		stack.push(t);
		TreeNode node=null;
		while(!stack.isEmpty()){
			node=stack.pop();
			list.add(node.val);
			if(node.left!=null){
				stack.push(node.left);
			}
			if(node.right!=null){
				stack.push(node.right);
			}
		}
		Collections.reverse(list);
		return list;
	}
	
}