Climbing Stairs

Desicription

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.

Example 1:

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Input: 2
Output:  2
Explanation:  There are two ways to climb to the top.
1. 1 step + 1 step
2. 2 steps

Example 2:

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Input: 3
Output:  3
Explanation:  There are three ways to climb to the top.
1. 1 step + 1 step + 1 step
2. 1 step + 2 steps
3. 2 steps + 1 step

Solution

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class Solution {
public:
    int climbStairs(int n) {
        long long a = 1;
        long long b = 1;
        while(n--)
            a = (b += a) - a;
        return a;
    }
};