I don't know why it handles return 0; which is not an exception after passing all conditional statements.

#include <stdio.h>
#include <stdlib.h>

typedef struct {
    int data;
    struct TreeNode *left, *right;
}TreeNode;

TreeNode n1 = { 1,NULL,NULL };
TreeNode n2 = { 4,NULL,NULL };
TreeNode n3 = { '*',&n1,&n2 };
TreeNode n4 = { 16,NULL,NULL };
TreeNode n5 = { 25,NULL,NULL };
TreeNode n6 = { '+',&n4,&n5 };
TreeNode n7 = { '+',&n3,&n6 };
TreeNode *exp = &n7;

// // Expression Evaluation Function
int evaluate(TreeNode *root)
{
    if (root == NULL)
        return 0;
    if (root->left == NULL && root->right == NULL)
        return root->data;
    else {
        int op1 = evaluate(root->left);
        int op2 = evaluate(root->right);
        printf("calculate %d %c %d.\n", op1, root->data, op2);
        switch (root->data) {
        case '+' :
            return op1 + op2;
        case '-':
            return op1 - op2;
        case '*':
            return op1 * op2;
        case '/':
            return op1 / op2;
        }
    }
    return 0;
}

int main(void)
{
    printf("Evaluation value is %d.\n", evaluate(exp));
    return 0;
}