fix

Author: lrvideckis

tests/library_checker_aizu_tests/trees/cd_count_paths_per_length.test.cpp

@@ -32,7 +32,7 @@ vector<ll> count_paths_per_length(vector<vi> adj) {
     sort(all(child_depths),
       [&](auto& x, auto& y) { return sz(x) < sz(y); });
     vector total_depth(1, 1.0);
-    for (auto& cnt_depth : child_depths) {
+    for (const auto& cnt_depth : child_depths) {
       auto prod = conv(total_depth, cnt_depth);
       rep(i, 1, sz(prod)) num_paths[i] += llround(prod[i]);
       total_depth.resize(sz(cnt_depth));