leetcode+ 上台阶，一步或者二步，DP

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class Solution {
public:
    int climbStairs(int n) {
        vector<int> dp;
        dp.push_back(0); dp.push_back(1); dp.push_back(2);
        for(int i=3;i<=n;i++) dp.push_back(dp[i-1]+dp[i-2]);
        return dp[n];
    }
};