实现二叉树的基本操作

先序遍历，中序遍历，后序遍历，求节点个数，求叶子节点的个数，求第k层节点的个数，查找节点。

import java.util.ArrayList;

public class TestBinaryTree {

    static class TreeNode {
        public char val;
        public TreeNode left;
        public TreeNode right;
        public TreeNode(char val) {
            this.val = val;
        }
    }

    public TreeNode creteTree() {
        TreeNode A = new TreeNode('A');
        TreeNode B = new TreeNode('B');
        TreeNode C = new TreeNode('C');
        TreeNode D = new TreeNode('D');
        TreeNode E = new TreeNode('E');
        TreeNode F = new TreeNode('F');
        TreeNode G = new TreeNode('G');
        TreeNode H = new TreeNode('H');

        A.left = B;
        A.right = C;
        B.left = D;
        B.right = E;
        E.right = H;
        C.left = F;
        C.right = G;
        return A;//根节点
    }


    // 前序遍历
    public void preOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        System.out.print(root.val+" ");
        preOrder(root.left);
        preOrder(root.right);
    }

    // 中序遍历
    void inOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        inOrder(root.left);
        System.out.print(root.val+" ");
        inOrder(root.right);
    }

    // 后序遍历
    void postOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        postOrder(root.left);
        postOrder(root.right);
        System.out.print(root.val+" ");
    }

    public static int count = 0;
    // 获取树中节点的个数         遍历思路：
    public void size1(TreeNode root){
        if(root == null) {
            return ;
        }
        count++;
        size1(root.right);
        size1(root.left);
    }

    public int size2(TreeNode root){
        if(root == null) {
            return 0;
        }
        //root不为空
        int tmp = size2(root.left) + size2(root.right)+1;
        return tmp;
    }


    public static int leafCount = 0;
    // 获取叶子节点的个数  遍历思路
    void getLeafNodeCount1(TreeNode root){
        if(root == null) {
            return;
        }
        if(root.left == null && root.right == null) {
            leafCount++;
        }
        getLeafNodeCount1(root.left);
        getLeafNodeCount1(root.right);
    }

    // 获取叶子节点的个数  子问题
    int getLeafNodeCount2(TreeNode root){
        if(root == null) {
            return 0;
        }
        if(root.left == null && root.right == null) {
            return 1;
        }
        int tmp = getLeafNodeCount2(root.left) + getLeafNodeCount2(root.right);
        return tmp;
    }


    // 获取第K层节点的个数
    int getKLevelNodeCount(TreeNode root,int k){
        if(root == null) {
            return 0;
        }
        if(k == 1) {
            return 1;
        }
        return getKLevelNodeCount(root.left,k-1)
                + getKLevelNodeCount(root.right,k-1);
    }

    // 获取二叉树的高度
    int getHeight(TreeNode root){
        if(root == null) {
            return 0;
        }
        int leftH = getHeight(root.left);
        int leftR = getHeight(root.right);
        return leftH > leftR ? leftH+1 : leftR+1;
    }

    // 检测值为value的元素是否存在
    TreeNode find(TreeNode root, int val){
        if(root == null) {
            return null;
        }
        if(root.val == val) {
            return root;
        }
        TreeNode ret = find(root.left,val);
        if(ret != null) {
            return ret;
        }
        ret = find(root.right,val);
        if(ret != null) {
            return ret;
        }
        return null;
    }

    // 判断一棵树是不是完全二叉树
    boolean isCompleteTree(TreeNode root) {
        return true;
    }
}

public class TestDemo {
    public static void main(String[] args) {
        TestBinaryTree testBinaryTree = new TestBinaryTree();
        TestBinaryTree.TreeNode root = testBinaryTree.creteTree();
        testBinaryTree.preOrder(root);
        System.out.println();

        testBinaryTree.inOrder(root);
        System.out.println();

        testBinaryTree.postOrder(root);
        System.out.println();

        System.out.println("=========节点的个数======");

        testBinaryTree.size1(root);

        System.out.println(TestBinaryTree.count);

        System.out.println("=========节点的个数======");
        System.out.println(testBinaryTree.size2(root));

        System.out.println("叶子节点的个数：");
        testBinaryTree.getLeafNodeCount1(root);
        System.out.println(TestBinaryTree.leafCount);

        System.out.println(testBinaryTree.getLeafNodeCount2(root));

        System.out.println("=============第K层==========");
        System.out.println(testBinaryTree.getKLevelNodeCount(root, 4));

        System.out.println("============查找某个数据========");
        TestBinaryTree.TreeNode ret = testBinaryTree.find(root,'p');
        try {
            System.out.println(ret.val);
        }catch (NullPointerException e) {
            System.out.println("没有你要找的数据！");
            e.printStackTrace();
        }
    }
}