leetCode 70. Climbing Stairs

You are climbing a stair case. It takes n steps to reach to the top.
Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?
Note: Given n will be a positive integer.

int climbStairs(int n) {
if (n<=0) return 0;
if (n==1) return 1;
if (n==2) return 2;

int one_step_before = 2;
int two_steps_before = 1;
int all_ways = 0;

for(int i=3; i<=n; i++) {
all_ways = one_step_before + two_steps_before;
two_steps_before = one_step_before;
one_step_before = all_ways;
}
return all_ways;
}