70. Climbing Stairs#

 1public int climbStairs(int n) {
 2  if (n == 1) return 1;
 3  if (n == 2) return 2;
 4
 5  int first = 1;
 6  int second = 2;
 7  int third = first + second;
 8
 9  for (int i = 3; i <= n; i++) {
10    third = first + second;
11
12    first = second;
13    second = third;
14  }
15
16  return third;
17}