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1601. Maximum Number of Achievable Transfer Requests

We have n buildings numbered from 0 to n - 1. Each building has a number of employees. It's transfer season, and some employees want to change the building they reside in.

You are given an array requests where requests[i] = [fromi, toi] represents an employee's request to transfer from building fromi to building toi.

All buildings are full, so a list of requests is achievable only if for each building, the net change in employee transfers is zero. This means the number of employees leaving is equal to the number of employees moving in. For example if n = 3 and two employees are leaving building 0, one is leaving building 1, and one is leaving building 2, there should be two employees moving to building 0, one employee moving to building 1, and one employee moving to building 2.

Return the maximum number of achievable requests.
解题重点：

1.

数据总量不大，可以回溯+枚举
2. 数组 delta 记录每一栋楼的员工变化量，变量 cnt 记录被选择的请求数量。
3. 对于第pos 个请求 [x,y]，如果选择该请求，那么就需要将 delta[x] 的值减 11， delta[y] 的值加 11，cnt 的值加 11
4. 修改 delta 的同时维护 delta 中的 0 的个数，记作zero，初始值为 n。如果 delta[x] 增加或减少前为 0，则将 zero 减 1；如果delta[x] 增加或减少后为 0，则将zero 加 1。

#赫海[超话]# [ NEWS｜NOW. ]
220128 NOW. 电台简介更新：

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