222. Count Complete Tree Nodes

Given a complete binary tree, count the number of nodes.

Note:

Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, 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.

Example:

Input:
    1
   / \
  2   3
 / \  /
4  5 6

Output: 6
  • Divide and conquer

public int countNodes(TreeNode root) {
    if(root == null) return 0;
    int l = getH(root.left), r = getH(root.right);
    if(l > r) { // r is complete
        return 1 + height2count(r) + countNodes(root.left);
    } else { // l is complete
        return 1 + height2count(l) + countNodes(root.right);
    }
}

public int getH(TreeNode root) {
    if(root == null) return -1;
    return 1 + getH(root.left);
}

public int height2count(int h) {
    return (int)Math.pow(2, h+1) - 1;
}