算法：DP算法

private static int climbStairs(int n) {
	if (n == 1) {
		return 1;
	}
	if (n == 2) {
		return 2;
	}
	int num1 = 1;
	int num2 = 2;
	for (int i = 2; i < n; i++) {
		int tmp = num1 + num2;
		num1 = num2;
		num2 = tmp;
	}
	return num2;
}