Count Complete Tree Nodes.java

/*
    Given a complete binary tree, count the number of nodes.
    
    Link: https://leetcode.com/problems/count-complete-tree-nodes/

    Example: None

    Solution: None
    
    Source: https://leetcode.com/discuss/81091/simple-java-solution-o-log-n-2-108-ms-with-explanation
*/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int countNodes(TreeNode root) {
        return root == null ? 0 : findLastIndex(root, 1);
    }
    private int lHeight(TreeNode node, int count) {
        return node == null ? count - 1 : lHeight(node.left, count + 1);
    }
    private int findLastIndex(TreeNode node, int currIndex) {
        if (node.left == null && node.right == null) {
        	return currIndex;
        }
        if (lHeight(node.left, 1) == lHeight(node.right, 1)) {
        	return findLastIndex(node.right, currIndex * 2 + 1);
        } else {
        	return findLastIndex(node.left, currIndex * 2);
        }
    }
}