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