You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

這道題說白了是遞迴，更好點是動態規劃，要求n，從n-1和n-2來。a[i] = a[i-1] + a[i-2]。本來想設一個數組，但由於只涉及到兩個數之間的求和，並且是相鄰的兩個數，就沒必要去設陣列了，只用兩個變數即可。

class Solution {
public:
    int climbStairs(int n) {
        if(n == 1 || n== 2)
            return n;
        int temp1 = 1;
        int temp2 = 2;
        for(int i = 3;i <= n;i++){
            int temp = temp1+temp2;
            temp1 = temp2;
            temp2 = temp;
        }
        return temp2;
    }
};