[C++] 3 lines
Intuition\n Describe your first thoughts on how to solve this problem. \n\n# Approach\n Describe your approach to solving the problem. \n\n# Complexity\n- Time 
# Intuition\n<!-- Describe your first thoughts on how to solve this problem. -->\n\n# Approach\n<!-- Describe your approach to solving the problem. -->\n\n# Complexity\n- Time complexity:\n<!-- Add your time complexity here, e.g. $$O(n)$$ -->\n\n- Space complexity:\n<!-- Add your space complexity here, e.g. $$O(n)$$ -->\n\n# Code\n```\nclass Solution {\npublic:\n    int firstDayBeenInAllRooms(vector<int>& v) {\n        long pre_sum[100001]{}, mod = 1000000007, n = v.size();\n        for(int i = 0; i < n; i++) pre_sum[i+1] = (pre_sum[i] + pre_sum[i] - pre_sum[v[i]] + 2 + mod)%mod;\n        return pre_sum[n-1];\n    }\n};\n```
['C++']