//

// Created by Chenguang Wang on 2024/1/20.

//

// https://leetcode.cn/problems/climbing-stairs/description/

class Solution {

public:

int climbStairs(int n) {

int n0 = 0, n1 = 0, result = 1;

for (int i = 1; i <= n; ++i) {

n0 = n1;

n1 = result;

result = n0 + n1;

}

return result;

}

int climbStairs1(int n) {

if (n == 0 || n == 1) {

return 1;

}

int a = 1; // f(n-2)

int b = 1; // f(n-1)

for (int i = 2; i <= n; ++i) {

int c = a + b;

a = b;

b = c;

}

return b;

}

int climbStairs2(int n) {

// 边界条件

if (n <= 1) {

return 1;

}

int prev1 = 1, prev2 = 1; // 初始状态：f(0) 和 f(1)

for (int i = 2; i <= n; ++i) {

int current = prev1 + prev2; // 当前状态

prev2 = prev1; // 更新 prev2

prev1 = current; // 更新 prev1

}

return prev1; // 最终结果

}

};