31. Climb stairs

70. climb stairs

Suppose you're climbing the stairs . need n You can reach the top of the building .

Every time you climb 1 or 2 A stair . How many different ways can you climb to the top of the building ？

** Be careful ：** Given n Is a positive integer .

Example 1：

 Input ： 2
 Output ： 2
 explain ：  There are two ways to climb to the top .
1.  1  rank  + 1  rank 
2.  2  rank 

Example 2：

 Input ： 3
 Output ： 3
 explain ：  There are three ways to climb to the top .
1.  1  rank  + 1  rank  + 1  rank 
2.  1  rank  + 2  rank 
3.  2  rank  + 1  rank 

Fibonacci sequence

class Solution {
    
    public int climbStairs(int n) {
    
        int a = 0;
        int b = 1;
        int res = 0;
        for(int i=1;i<=n;i++){
    
            int c = a;
            a = b;
            b = c +b;
        }

        return b;
    }
}

class Solution {
    
    public int climbStairs(int n) {
    
        int[] f = new int[n+1];
        f[0] = 1;
        f[1] = 1;
        for(int i=2;i<=n;i++){
    
            f[i]=f[i-1]+f[i-2];
        }

        return f[n];
    }
}