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Language/Type: C++ binary trees pointers recursion
Author: Marty Stepp (on 2016/06/16)

Write a member function named countLeftNodes that could be added to the BinaryTree class. Your function should return the number of left children in the tree. A left child is a node that appears as the root of the left-hand subtree of another node. For example, the tree below has 4 left children (the nodes storing the values 5, 1, 4, and 7):

   / \
  5   2
 /   / \
1   4   6

Constraints: Do not construct any new BinaryTreeNode objects in solving this problem (though you may create as many BinaryTreeNode* pointer variables as you like). Do not use any auxiliary data structures to solve this problem (no array, vector, stack, queue, string, etc). Your member function should not modify the tree's state; the state of the tree should remain constant with respect to your function. You should declare the function to indicate this to the caller.

Write the member function as it would appear in BinaryTree.cpp. You do not need to declare the function header that would appear in BinaryTree.h. Assume that you are adding this method to the BinaryTree class as defined below:

class BinaryTree {
    BinaryTreeNode* root;   // NULL for an empty tree
    your code goes here;

struct BinaryTreeNode {
    int data;
    BinaryTreeNode* left;
    BinaryTreeNode* right;
Type your solution here:

This is a member function problem. Submit a member function that will become part of an existing C++ class. You do not need to write the complete class, just the member function described in the problem.

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