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?

class Solution {public:    int climbStairs(int n) {        if(n<=0)         return 0;        if(n==1)         return 1;        if(n==2)         return 2;        int i1=1,i2=2,i3=0;        n-=2;        while(n)        {            --n;            i3=i1+i2;            i1=i2;            i2=i3;        }        return i3;    }};