70. Climbing Stairs

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?

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需要清楚的是n-1和n-2时，都只有一种方法爬上去

n-1时爬一阶

n-2时爬二阶。

感觉不是很好理解。需要再向别人请教。

代码：

class Solution {
public:
    int climbStairs(int n) {
        vector<int> dp(n+1);
        dp[1]=1;
        dp[2]=2;
        for(int i=3;i<n+1;++i)
        dp[i]=dp[i-1]+dp[i-2];
        return dp[n];
    }
};