70. Climbing Stairs（LeetCode）

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?

Note: Given n will be a positive integer.

class Solution {
public:
    int climbStairs(int n) {
      int sum = 0;
        if (n <= 1)
            return 1;
        if (n == 2)
            return 2;
        int a = 1, b = 2;
        for (int i = 3; i <= n; i++)
        {
            sum = a + b;
            a = b;
            b = sum;
            sum=0;
        }
        return b;
    }
};