Path Sum
	
	/**
	 * Given a binary tree and a sum, determine if the tree has a
root-to-leaf
	 * path such that adding up all the values along the path equals the
given
	 * sum.
	 * 
	 * For example: Given the below binary tree and sum = 22,
	 * 
	 *  	  5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1
	 * 
	 * return true, as there exist a root-to-leaf path 5->4->11->2 which
sum is 22.
	 * 
	 * */