[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?

Method:
This is a recursion question. M(n)=M(n-1)+M(n-2), but it will cause timeout error if we simply write this recursion equation.

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