70. Climbing Stairs

1.描述

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.

2.分析

递推
f(1) = 1;
f(2) = 2;
f(n) = f(n-2) + f(n-1);

3.代码

int climbStairs(int n) {
    if (1 == n) return 1;
    if (2 == n) return 2;
    
    int f1 = 1;
    int f2 = 2;
    int f3 = 0;
    for (unsigned int i = 3; i <= n; ++i) {
        f3 = f1 + f2;
        f1 = f2;
        f2 = f3;
    }
    return f3;
}