A binary tree structure is defined as follows: struct Tree { int item; struct Tree * left_child; struct Tree * right_child; } If a left child or right child does not exist, the corresponding member variable is set to NULL. Write a recursive function int positive(struct Tree * tree) that returns 0 if there is at least one node that has a negative integer and 1 otherwise (all the nodes have integers that are greater than or equal to 0). (Your program should not crash.)