CountCompleteTreeNodes.cpp

// Source : https://leetcode.com/problems/count-complete-tree-nodes/
// Author : Hao Chen
// Date   : 2015-06-12

/********************************************************************************** 
 * 
 * Given a complete binary tree, count the number of nodes.
 * 
 * Definition of a complete binary tree from Wikipedia:
 * http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees
 *
 * 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.
 *               
 **********************************************************************************/


/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    //return -1 if it is not.
    int isCompleteTree(TreeNode* root) {
        if (!root) return 0;

        int cnt = 1;
        TreeNode *left = root, *right = root;
        for(; left && right; left=left->left, right=right->right) {
            cnt *= 2;
        }
      
        if (left!=NULL || right!=NULL) {
            return -1;
        }
        return cnt-1;
    }

    int countNodes(TreeNode* root) {
        int cnt = isCompleteTree(root);
        if (cnt != -1) return cnt;
        int leftCnt = countNodes(root->left);
        int rightCnt = countNodes(root->right);
        return leftCnt + rightCnt + 1;
    }
};