This problem is a Fibonacci problem.

F(n)=F(n-1)+F(n-2);

Solving this problem by recursion ,we will do a lot of same recursion.

Example:

F(10)=F(9)+F(8);

F(9)=F(8)+F(7);

we calculate F(8) twice,when n is large,this will increase as a rate of n's exponent.

So a more efficient way to solve this problem is from Bottom to Top.

Calculate F(0) ,F(1);

then F(2).........

//F(n) = F(n-1) + F(n-2)class Solution {public:    int climbStairs(int n) {        //if(n<=2) return n; //0,1        int f0 = 0, f1=1, steps =0;        for(int i=0; i