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1我们先来模拟创建一个二叉树

public class TestBinaryTreee {
    static class TreeNode{
        public char val;
        public  TreeNode left;
        public  TreeNode right;
        public TreeNode(char val) {
            this.val = val;
        }
    }
    public TreeNode creatTree(){
        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;
        C.left = F;
        C.right = G;
        E.right = H;
        return A;
        //就是根节点
    }
}

2分别实现它的前中后序遍历

1前序遍历

// 前序遍历
    void preOrder(TreeNode root){
        if (root == null){
            return;
        }
        System.out.print(root.val+" ");
        //递归遍历左子树
        preOrder(root.left);
        //第归遍历右子树
        preOrder(root.right );
    }

 TestBinaryTreee testBinaryTreee = new TestBinaryTreee();
        testBinaryTreee.creatTree();
        TestBinaryTreee.TreeNode root = testBinaryTreee.creatTree();
        testBinaryTreee.preOrder(root);
        System.out.println();

2中序遍历

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

    }

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

3后序遍历

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

    }

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

3 获取树中节点的个数

 //求有多少个节点
1:
    public int size(TreeNode root){
        if (root == null){
            return 0;
        }
        int ret = size(root.left)+
                size(root.right)+1;
        return ret;
    }
2:
    public static int nodeSize;
    public void size2(TreeNode root){
        if (root == null){
            return ;
        }
        nodeSize++;
        size2(root.left);
        size2(root.right);
    }

 System.out.println(testBinaryTreee.size(root));

 testBinaryTreee.size2(root);
        System.out.println(TestBinaryTreee.nodeSize);

4获取叶子节点的个数

//求叶子结点个数
1:
    public int getLeafNodeCount(TreeNode root){
        if (root == null){
            return 0;
        }
        if(root.left == null && root.right ==null){
            return 1;
        }
        return getLeafNodeCount(root.left)+
                getLeafNodeCount(root.right);
    }
2:
    public int leafSize;
    public void getLeafNodeCount2(TreeNode root){
        if (root == null){
            return ;
        }
        if(root.left == null && root.right ==null){
           leafSize++;
        }
        getLeafNodeCount2(root.left);
        getLeafNodeCount2(root.right);
    }

 System.out.println(testBinaryTreee.getLeafNodeCount(root));
        testBinaryTreee.getLeafNodeCount2(root);
        System.out.println(testBinaryTreee.leafSize);

5 获取第K层节点的个数

 //获取第k层节点个数
    public 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);
    }

System.out.println(testBinaryTreee.getKLevelNodeCount(root, 3));

 

6获取二叉树的高度

 //获取二叉树高度
    public int getHeight(TreeNode root){
        if(root == null){
            return 0;
        }
        int leftHeight = getHeight(root.left);
        int rightHeight = getHeight(root.right);
        return leftHeight > rightHeight ?
                leftHeight+1 : rightHeight+1;

    }

int height = testBinaryTreee.getHeight(root);
        System.out.println(height);

7检测值为value的元素是否存在

 // 检测值为val的元素是否存在
    public TreeNode find(TreeNode root,char 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;
    }

TestBinaryTreee.TreeNode retN = testBinaryTreee.find(root, 'E');
        System.out.println(retN.val);