public Percolation(int N){
		// create N-by-N grid, with all sites blocked
		id = new int[(N * N) + 1];
		op = new int[N * N];
		length = N;

		for (int i = 0; i < id.length; i++) {
			if (i < N) {
				id[i] = Integer.MIN_VALUE;
				op[i] = 1;
			} else if (i == id.length) {
				id[i] = Integer.MAX_VALUE;
			} else {
				id[i] = i;
				op[i] = 0;
			}
		}
	}