LeetCode - 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?

class Solution {
public:
int climbStairs(int n) {
if(n<=0){
return 0;
}
if(n<=2){
return n;
}
int a=1,b=2,c;
for(int i=3;i<=n;i++){
c=b;
b+=a;
a=c;
}
return b;
}
};