二叉树先序中序后序遍历实现

一、递归实现

import java.util.*;

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}*/
public class TreeToSequence {
    public int[][] convert(TreeNode root) {
        // write code here
        //函数返回值是一个二维数组，此二维数组储存了三种遍历方式的遍历结果
        //首先创建三个list结构用来存储遍历出的树型结构
        List<Integer> preorderList = new ArrayList<Integer>();
        List<Integer> inorderList = new ArrayList<Integer>();
        List<Integer> postorderList = new ArrayList<Integer>();
        
        //调用遍历函数
        transPreorder(root, preorderList);
        transInorder(root, inorderList);
        transPostorder(root, postorderList);
        //创建二维数组
        int length = preorderList.size();
        int[][] result = new int[3][length];
        
        //取出遍历结果
        for(int i = 0; i < length; i++) {
            result[0][i] = preorderList.get(i);
        }
 
        for(int i = 0; i < length; i++) {
            result[1][i] = inorderList.get(i);
        }
 
        for(int i = 0; i < length; i++) {
            result[2][i] = postorderList.get(i);
        }
 
        return result;
    }
    //先序遍历
    public void transPreorder(TreeNode node, List<Integer> list){
        if(node==null){
            return;
        }
        list.add(node.val);
        transPreorder(node.left,list);
        transPreorder(node.right,list);
        
    }
    //中序遍历
    public void transInorder(TreeNode node, List<Integer> list){
        if(node==null){
            return;
        }
        transInorder(node.left,list);
        list.add(node.val);
        transInorder(node.right,list);
    }
    //后序遍历
    public void transPostorder(TreeNode node, List<Integer> list){
        if(node==null){
            return;
        }
        transPostorder(node.left,list);
        transPostorder(node.right,list);
        list.add(node.val);
    }
}

二、非递归实现

import java.util.*;

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}*/
public class TreeToSequence {
    public int[][] convert(TreeNode root) {
        // write code here
        List<Integer> pre = new ArrayList<Integer>();
        List<Integer> in = new ArrayList<Integer>();
        List<Integer> post = new ArrayList<Integer>();
        preOrder(root, pre);
        inOrder(root, in);
        postOrder(root, post);
        int[][] res = new int[3][pre.size()];
        for(int i = 0; i < pre.size(); i++){
            res[0][i] = pre.get(i);
            res[1][i] = in.get(i);
            res[2][i] = post.get(i);
        }
        return res;
    }
    
    //先序遍历
    public void preOrder(TreeNode root,List<Integer> list){
        if(root == null)
            return;
        //创建一个栈结构,并将根节点入栈
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        //当栈不为空时，弹出栈顶元素并记录，如果该元素的右子树不为空，那么将他的右子树入栈，继续，如果他的左子树不为空，入栈
        while(!stack.isEmpty()){
            TreeNode cur = stack.pop();
            list.add(cur.val);
            if(cur.right!=null)
                stack.push(cur.right);
            if(cur.left != null)
                stack.push(cur.left);
        }
    }
    
    //中序遍历
    public void inOrder(TreeNode root,List<Integer> list){
        if(root == null)
            return;
        //创建一个栈结构，将根节点作为当前结点
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode cur = root;
        //当栈不为空，并且当前结点不为空时
        while(!stack.isEmpty() || cur != null){
            //如果当前结点不为空，则将该节点入栈并且将指针移向左子树
            if(cur != null){
                stack.push(cur);
                cur = cur.left;
            }else{
                //如果为空则弹出栈顶元素，并且将指针移向出栈结点的右子树
                cur = stack.pop();
                list.add(cur.val);
                cur = cur.right;
            }
        }
    }
    
    //后序遍历
    public void postOrder(TreeNode root,List<Integer> list){
        if(root == null)
            return;
        //创建一个栈结构，将根节点作为当前结点
        Stack<TreeNode> stack = new Stack<TreeNode>();
        //创建一个栈顶结点和一个标记结点
        TreeNode h = root, c = null;
        stack.push(root);
        while(!stack.isEmpty()){
            c = stack.peek();
            //如果标记结点的左子树和右子树都不为空，并且标记结点的左子树和栈顶结点不同，说明标记结点的左子树和右子树都没有处理过
            if(c.left != null && (c.left != h && c.right != h)){
                stack.push(c.left);
                c = c.left;
            }else if(c.right != null && c.right != h){  //如果c的右子树不等于栈顶结点，说明右子树没有处理过
                stack.push(c.right);
                c = c.right;
            }else{                              //如果都处理了，则弹出栈顶元素并且将当前标记的节点置为栈顶元素。
                h = c;
                TreeNode tmp = stack.pop();
                list.add(tmp.val);
            }
        }
    }
}