Link: https://leetcode.cn/problems/count-complete-tree-nodes/

Question

difficulty: mid
adj diff: 3

Given the root of a complete binary tree, return the number of the nodes in the tree.

According to Wikipedia, every level, except possibly the last, is completely filled in a complete binary tree, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

Design an algorithm that runs in less than O(n) time complexity.

Example 1:

Input: root = [1,2,3,4,5,6]
Output: 6

Example 2:

Input: root = []
Output: 0

Example 3:

Input: root = [1]
Output: 1

Constraints:

	The number of nodes in the tree is in the range [0, 5 * 104].
	0 <= Node.val <= 5 * 104
	The tree is guaranteed to be complete.

判断左右 depth，是否相等。

Code

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public int countNodes(TreeNode root) {
    if (root == null) return 0;
    int depthLeft = depth(root.left);
    int depthRight = depth(root.right);
    if (depthLeft == depthRight) {
        // left tree is complete
        return (int) Math.pow(2, depthLeft) + countNodes(root.right);
    } else {
        return 1 + countNodes(root.left) + countNodes(root.right);
    }
}

private int depth(TreeNode node) {
    if (node == null) return 0;
    return depth(node.left) + 1;
}