二叉树的简单学习

【概念】

先序： 根、左、右

中序:   左、根、右

后序:   左、右、根

【代码】

package com.company;
import java.util.*;
import java.util.stream.Collectors;

/**
 * 二叉树节点
 */
class TreeNode {
    int data;
    TreeNode left;
    TreeNode right;

    TreeNode(int data){
        this.data = data;
    }

    @Override
    public String toString() {
        return "TreeNode{" +
                "data=" + data +
                ", left=" + left +
                ", right=" + right +
                '}';
    }
}

public class threeSum {
    public static void main(String[] args) {
        LinkedList<Integer> list = new LinkedList<>(Arrays.
                asList(new Integer[]{3,2,9,null,null,10,null,
                        null,8,null,4}));
        TreeNode treeNode = createBinaryTree(list);
        System.out.println(treeNode.toString());
        System.out.println("前序:");
        preOrderTraveral(treeNode);
        System.out.println("\n中序:");
        inOrderTraveral(treeNode);
        System.out.println("\n后序:");
        postOrderTraveral(treeNode);

    }


    public static TreeNode createBinaryTree(LinkedList<Integer> inputList){
        TreeNode node = null;

        if(inputList == null || inputList.isEmpty()) {
            return null;
        }

        Integer data = inputList.removeFirst();
        if(data != null) {
            node = new TreeNode(data);
            node.left = createBinaryTree(inputList);
            node.right = createBinaryTree(inputList);
        }
        return node;
    }

    // 先序: 根 左 右
    public static void preOrderTraveral(TreeNode node) {
        if (node == null) {
            return;
        }

        System.out.print(node.data + " ");
        preOrderTraveral(node.left);
        preOrderTraveral(node.right);
    }

    // 中序: 左 根 右
    public static void inOrderTraveral(TreeNode node) {
        if (node == null) {
            return;
        }
        inOrderTraveral(node.left);
        System.out.print(node.data + " ");
        inOrderTraveral(node.right);
    }

    // 后序: 左、右、根
    public static void postOrderTraveral(TreeNode node) {
        if (node == null) {
            return;
        }
        inOrderTraveral(node.left);
        inOrderTraveral(node.right);
        System.out.print(node.data + " ");
    }

}