Java实现二叉树的基本操作详解

static class Treenode {
        public char val;//数值
        public TreeNode left;//左子树引用
        public TreeNode right;//右子树引用

        public TreeNode(char val) {
            this.val = val;
        }
    }

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

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

    //获取树中节点的个数,遍历计数法
    public static int nodeSize;
    public int size(TreeNode root) {
        //先将nodeSzie置为0
        nodeSize = 0;
        sizefunc(root);
        return nodeSize;
    }
    public void sizefunc(TreeNode root) {
        if(root == null) {
            return;
        }
        nodeSize++;
        sizefunc(root.left);
        sizefunc(root.right);
    }

    //获取树中节点的个数,子问题思想
    public int size2(TreeNode root) {
        if(root == null) {
            return 0;
        }
        return size2(root.left) + size2(root.right) + 1;
    }

    //获取叶子节点的个数,子问题思想
    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);
    }
    //获取叶子节点的个数,遍历计数法
    public static int leafSize;
    public int getLeafNodeCount2(TreeNode root){
        leafSize = 0;
        getLeafNodeCount2func(root);
        return leafSize;
    }
    public void getLeafNodeCount2func(TreeNode root) {
        if(root == null) {
            return;
        }
        if(root.left == null && root.right == null) {
            leafSize++;
        }
        getLeafNodeCount2func(root.left);
        getLeafNodeCount2func(root.right);
    }

public int getKLevelNodeCount(TreeNode root, int k) {
        if(root == null || k <= 0) {
            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
    public int getHeight(TreeNode root) {
        if(root == null) {
            return 0;
        }
        int leftHight = getHeight(root.left);
        int rightHight = getHeight(root.right);
        return leftHight>rightHight ? leftHight+1 : rightHight+1;
    }

//检测值为val的元素是否存在
    public boolean find(TreeNode root, char val) {
        if(root == null) {
            return false;
        }
        if(root.val == val) {
            return true;
        }
        boolean ret1 = find(root.left, val);
        if(ret1){
            return true;
        }
        boolean ret2 = find(root.right, val);
        if(ret2){
            return true;
        }
        return false;
    }

//判断一棵树是不是完全二叉树
    public boolean isCompleteTree(TreeNode root) {
        if(root == null) {
            return true;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            if(cur == null) {
                break;
            }
            queue.offer(cur.left);
            queue.offer(cur.right);
        }
        //判断队列中是否有不为空的元素
        int size = queue.size();
        while(size != 0) {
            size--;
            if(queue.poll() != null) {
                return false;
            }
        }
        return true;
    }

//层序遍历
    public void levelOrder(TreeNode root) {
        if(root == null) {
            return;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            System.out.print(cur.val+" ");
            if(cur.left != null) {
                queue.offer(cur.left);
            }
            if(cur.right != null) {
                queue.offer(cur.right);
            }
        }
     }