Union Array Tree Calculate

#include <stdio.h>

typedef union {
	char op;
	int opnd;
	int is_thread;
}element;

void preorder(element *c, int i) {
	if (c[i].op != NULL) {
		if (c[i].op == '+' || c[i].op == '*' || c[i].op == '/')
			printf("%c ", c[i].op);
		else
			printf("%d ", c[i].opnd);
		preorder(c, 2 * i);
		preorder(c, 2 * i + 1);
	}
}

void inorder(element *c, int i) {
	if (c[i].op != NULL) {
		inorder(c, 2 * i);
		if (c[i].op == '+' || c[i].op == '*' || c[i].op == '/')
			printf("%c ", c[i].op);
		else
			printf("%d ", c[i].opnd);
		inorder(c, 2 * i + 1);
	}
}

void postorder(element *c, int i) {
	if (c[i].op != NULL) {
		postorder(c, 2 * i);
		postorder(c, 2 * i + 1);
		if (c[i].op == '+' || c[i].op == '*' || c[i].op == '/')
			printf("%c ", c[i].op);
		else
			printf("%d ", c[i].opnd);
	}
}

int power(int a, int b) { // 제곱 함수
	if (b == 0)
		return 1;
	else
		return a * power(a, b - 1);
}


int calc(char operator, int leftopnd, int rightopnd) {
	switch (operator) {
	case '+':
		return leftopnd + rightopnd;
	case '-':
		return leftopnd - rightopnd;
	case '*':
		return leftopnd * rightopnd;
	case '/':
		return leftopnd / rightopnd;
	case '%':
		return leftopnd % rightopnd;
	case '^':
		return power(leftopnd, rightopnd);
	}
}

int eval(element *t, int i) {
	if (t[i].op == NULL)
		return 0;
	else if (t[2 * i].opnd == NULL && t[2 * i + 1].opnd == NULL)
		return t[i].opnd;
	else {
		int leftopnd = eval(t, 2 * i);
		int rightopnd = eval(t, 2 * i + 1);
		return calc(t[i].op, leftopnd, rightopnd);
	}
}

int get_count(element *t, int i) {
	int count = 0;

	if (t[i].op != 0) {
		count = 1 + get_count(t, 2 * i) + get_count(t, 2 * i + 1);
	}
	return count;
}


int main() {
	element c[16] = { 0,'+','*','/',3,2,30,6 };
	printf("중위 표현: ");
	preorder(c, 1);
	printf("\n전위 표현: ");
	inorder(c, 1);
	printf("\n후위 표현: ");
	postorder(c, 1);
	printf("\n계산: %d", eval(c, 1));
	printf("\n노드 개수: %d\n", get_count(c, 1));
	system("pause");
}