实现二叉树基本操作

二叉树基本操作

1. 获取二叉树中节点个数

计数器思路:

 public int nodeNum;
    public void size1(TreeNode root) {
        if (root == null) {
            return;
        }
        this.nodeNum++;
        size1(root.left);
        size1(root.right);
    }

子问题思路:

每颗树节点个数 = 左子树节点个数 + 右子树节点个数 + 1(根)

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

 

2. 获取二叉树叶子节点个数

计数器思路:

 public int leafNodeNum;
    public void getLeafNodeCount1(TreeNode root) {
        if (root == null) {
            return;
        }
        if (root.left == null && root.right == null) {
            this.leafNodeNum++;
        }
        getLeafNodeCount1(root.left);
        getLeafNodeCount1(root.right);
    }

子问题思路:

每颗树叶子节点个数 = 左子树叶子节点个数 + 右子树节点个数

 

public int getLeafNodeCount2(TreeNode root) {
        if (root == null) {
            return 0;
        }
        if (root.left == null && root.right == null) {
            return 1;
        }
        return getLeafNodeCount2(root.left)
                + getLeafNodeCount2(root.right);
    }

 3. 获取第K层节点的个数

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);
}

4. 获取二叉树的高度

思路:

每颗树的高度 = 左子树与右子树高度的最大值 + 1(根)

 

 public int getHeight(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int leftHeight = getHeight(root.left);
        int rightHeight = getHeight(root.right);
        return Math.max(leftHeight,rightHeight) + 1;
}

 5. 检测值为value的元素是否存在

public TreeNode find(TreeNode root, int val) {
        if (root == null) {
            return null;
        }
        if (root.val == val) {
            return root;
        }
        TreeNode leftVal = find(root.left,val);
        if (leftVal != null) {
            return leftVal;
        }
        TreeNode rightVal = find(root.right,val);
        if (rightVal != null) {
            return rightVal;
        }
        return null;
}

6.  层序遍历

什么层序遍历 ? 从上到下 从左到右依次打印

如何非递归实现 层序遍历 ? 使用队列结构

public void levelOrder(TreeNode root) {
        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);
            }
        }
 }

 栈能否实现层序遍历 ?

7. 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) {
                queue.offer(cur.left);
                queue.offer(cur.right);
            }else {
                break;
            }

        }
        while (!queue.isEmpty()) {
            if (queue.poll() != null) {
                return false;
            }
        }
        return true;
}