Given a complete binary tree, count the number of nodes.
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.
public int countNodes(TreeNode root) { if(root == null) return 0; int hl = 0, hr = 0; TreeNode l = root, r = root; while(l != null) { hl++; l = l.left; } while(r != null) { hr++; r = r.right; } if(hl == hr) { return (int)Math.pow(2, hl)-1; } return 1+countNodes(root.left)+countNodes(root.right); }