差分约束的md
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# 差分约束
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* https://vjudge.net/contest/509525#discuss
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### 题意
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不明所以
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### 做法
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* x1-x2<=w
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* x1<=x2+w
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* dist[v]<=dist[u]+w
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* add(x2,x1,w);
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* x1-x2>=1? add(x1,x2,-1);
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* 最短路->最大值,反之亦然
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![-1的示意图](-1.png)
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### 关键词
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数学,最短路,spfa
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### 易错点
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* 隐藏条件
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* 构图
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### 工具箱
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* PPT
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