Merge pull request #156 from yosupo06/patch/constraints

clarify constraints just in case

Author: yosupo06

document_en/mincostflow.md

@@ -76,6 +76,7 @@ It returns $g$ as the list of the changepoints, that satisfies the followings.
 Let $x$ be the maximum cost among all edges.
 
 - $s \neq t$
+- $0 \leq s, t \lt n$
 - You can't call `min_cost_slope` or `min_cost_max_flow` multiple times.
 - The total amount of the flow is in `Cap`.
 - The total cost of the flow is in `Cost`.

document_ja/mincostflow.md

@@ -76,6 +76,7 @@ vector<pair<Cap, Cost>> graph.slope(int s, int t, Cap flow_limit);
 辺のコストの最大を $x$ として
 
 - $s \neq t$
+- $0 \leq s, t \lt n$
 - `min_cost_slope`や`min_cost_max_flow`を合わせて複数回呼んだときの挙動は未定義
 - `s`から`t`へ流したフローの流量が `Cap` に収まる。
 - 流したフローのコストの総和が `Cost` に収まる。