1+23-4+56+7+8+9 = 100

gistfile1.c

#include <stdio.h>

void recursive(int level, int* op) {
	if(level == 8) {
		int result = 0;
		int lastOp = 1;
		int stack = 0;
		for(int i = 0; i < 9; i++) {
			int num = i + 1;
			stack *= 10;
			stack += num;
			if(num <= 8) {
				if(op[i] == 2) {
					continue;
				}
				if(op[i] == 0) {
					result += lastOp * stack;
					lastOp = 1;
					stack = 0;
				}
				if(op[i] == 1) {
					result += lastOp * stack;
					lastOp = -1;
					stack = 0;
				}
			} else {
				result += lastOp * stack;
			}
		}
		if(result == 100) {
			for(int i = 0; i < 8; i++) {
				printf("%d", i + 1);
				switch(op[i]) {
					case 0: printf("+"); break;
					case 1: printf("-"); break;
					case 2: break;
				}
			}
			printf("9=100\n");
		}
		return;
	}
	for(int i = 0; i < 3; i++) {
		op[level] = i;
		recursive(level + 1, op);
	}
}
int main(void) {
	int op[8] = {0,0,0,0,0,0,0,0};
	recursive(0, op);
	return 0;
}