LeetCode Brushing Notes_70. Climbing the stairs

The topic is from LeetCode

70. Climb stairs

Other solutions or source code can be accessed: tongji4m3

description

Suppose you are climbing stairs. It takes n steps to reach the top of the building.

You can climb 1 or 2 steps each time. How many different ways do you have to climb to the top of a building?

Note: Given n is a positive integer.

Example 1:

输入： 2
输出： 2
解释： 有两种方法可以爬到楼顶。
1.  1 阶 + 1 阶
2.  2 阶

Example 2:

输入： 3
输出： 3
解释： 有三种方法可以爬到楼顶。
1.  1 阶 + 1 阶 + 1 阶
2.  1 阶 + 2 阶
3.  2 阶 + 1 阶

Ideas

It’s actually the Fibonacci sequence

Code

public int climbStairs(int n)
{
    
    
    if(n==1 || n==2) return n;//正整数,不用考虑这么多

    int first = 1, second = 2;//初始化为爬1楼,2楼的方法数量
    for (int i = 3; i <= n; i++)//i代表楼梯数
    {
    
    
        int temp = first + second;
        first = second;
        second = temp;
    }
    return second;
}

Complexity analysis

time complexity

$O\left(N\right)$

Space complexity

$O\left(1\right)$