DP-LeetCode-70. 爬楼梯

f[0] = 1;
f[2] = 2;
f[n] = f[n-1] + f[n-2]

class Solution {
public:
    int climbStairs(int n) {
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return n;
        }
        n -= 2;
        int a = 1;
        int b = 2;
        int c = 0;
        while (n--) {
            c = a + b;
            a = b;
            b = c;
        }
        return c;
    }
};