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

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 2^h nodes inclusive at the last level h.

思路：
我們只需要看左子樹是不是滿的，如果滿，我們遞歸分析右子樹；如果左子樹不滿，我們遞歸分析左子樹。

代碼：

int countNodes(TreeNode* root) {
        if(root == NULL) return 0;
        int num = 1;
        TreeNode *curL = root -> left, *curR = root -> left;
        while(curR){
            curL = curL -> left;
            curR = curR -> right;
            num = num << 1;
        }
        return num + ((!curL) ? countNodes(root -> right) : countNodes(root -> left));
    }

注：使用了位運算，效率高。