最全的树的遍历算法（递归和非递归）

树的算法如下：非递归的算法中，中序和后续遍历稍微难写一些 ，需要多练。其余的基本是模板方式，多横向对比感受下。

import java.util.Stack;

public class SearchTree {

    public static void main(String[] args) {
        BinaryTreeNode root = new BinaryTreeNode(1);
        root.left = new BinaryTreeNode(2);
        root.left.left = new BinaryTreeNode(4);
        root.left.left.left = new BinaryTreeNode(8);
        root.left.left.right = new BinaryTreeNode(9);
        root.left.right = new BinaryTreeNode(5);
        root.left.right.left = new BinaryTreeNode(10);
        root.right = new BinaryTreeNode(3);
        root.right.left = new BinaryTreeNode(6);
        root.right.right = new BinaryTreeNode(7);

        System.out.println("先序遍历的递归和非递归对比：");
        noRecursionPreOrder(root);
        System.out.println();
        recursionPreOrder(root);
        System.out.println();

        System.out.println("中序遍历的递归和非递归对比：");
        noRecursionMidOrder(root);
        System.out.println();
        recursionMidOrder(root);
        System.out.println();

        System.out.println("后序遍历的递归和非递归对比：");
        noRecursionPostOrder(root);
        System.out.println();
        recursionPostOrder(root);
        System.out.println();

    }

    // 递归版 --> 二叉树(先序遍历)
    public static void noRecursionPreOrder(BinaryTreeNode node) {
        // 1. 递归终止条件
        if (node == null) return;
        // 2. 处理当前层逻辑
        System.out.print(node.data + " ");
        // 3. 下挖一层
        noRecursionPreOrder(node.left);
        noRecursionPreOrder(node.right);
    }

    // 非递归版 --> 二叉树(先序遍历)
    public static void recursionPreOrder(BinaryTreeNode root) {
        if (root == null) {
            return;
        }
        Stack<BinaryTreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            BinaryTreeNode node = stack.pop();
            System.out.print(node.data + " ");
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
    }

    // 递归版 --> 二叉树(中序遍历)
    public static void noRecursionMidOrder(BinaryTreeNode node) {
        if (node == null) return;
        noRecursionMidOrder(node.left);
        System.out.print(node.data + " ");
        noRecursionMidOrder(node.right);
    }

    // 非递归版 --> 二叉树(中序遍历)
    public static void recursionMidOrder(BinaryTreeNode node) {
        if (node == null) return;
        Stack<BinaryTreeNode> stack = new Stack<>();
        while (node != null || !stack.isEmpty()) {
            if (node != null) {
                stack.push(node);
                node = node.left;
            } else {
                node = stack.pop();
                System.out.print(node.data + " ");
                node = node.right;
            }
        }
    }

    // 递归版 --> 二叉树(后序遍历)
    public static void noRecursionPostOrder(BinaryTreeNode node) {
        if (node == null) return;
        noRecursionPostOrder(node.left);
        noRecursionPostOrder(node.right);
        System.out.print(node.data + " ");
    }

    // 非递归版 --> 二叉树(后序遍历)
    public static void recursionPostOrder(BinaryTreeNode node) {
        if (node == null) {
            return;
        }
        Stack<BinaryTreeNode> stack1 = new Stack<>();
        Stack<BinaryTreeNode> stack2 = new Stack<>();
        stack1.push(node);
        while (!stack1.isEmpty()) {
            node = stack1.pop();
            stack2.push(node);
            if (node.left != null) stack1.push(node.left);
            if (node.right != null) stack1.push(node.right);
        }
        while (!stack2.isEmpty()) {
            System.out.print(stack2.pop().data + " ");
        }
    }

    // 递归版 --> 深度遍历
    public static void dfs04(TreeNode node) {
        if (node == null) return;
        System.out.print(node.data + " ");
        for (int i = 0; i < node.childNodes.size(); i++) {
            dfs04(node.childNodes.get(i));
        }
    }
}

class BinaryTreeNode {
    int data; // 数据域
    BinaryTreeNode left;// 左子节点
    BinaryTreeNode right; // 右子节点

    public BinaryTreeNode(int data) {
        this.data = data;
    }
}