【DP】70.爬楼梯

题目

Python

class Solution:
    def climbStairs(self, n: int) -> int:
        if n == 1:
            return 1
            
        pre1 = 1
        pre2 = 2
        ans = pre2
        for i in range(3, n+1):
            ans = pre1 + pre2
            pre1 = pre2
            pre2 = ans
        return ans

Java

法1：dp

相当于计算斐波那契数列

class Solution {
    public int climbStairs(int n) {
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 2;
        }
        int pre0 = 1, pre1 = 2, res = 0;
        while (n - 2 > 0) {
            res = pre0 + pre1;
            pre0 = pre1;
            pre1 = res;
            --n;
        }

        return res;
    }
}