CP Solver: small code cleanups.

src/constraint_solver/demon_profiler.cc

@@ -380,7 +380,7 @@ class DemonProfiler : public PropagationMonitor {
       runtimes.push_back(demon_time);
     }
     // Compute mean.
-    if (runtimes.size()) {
+    if (!runtimes.empty()) {
       *mean_demon_runtime = (1.0L * *total_demon_runtime) / runtimes.size();
 
       // Compute median.

src/constraint_solver/expr_array.cc

@@ -2356,7 +2356,8 @@ class PositiveBooleanScalProdEqCst : public Constraint {
 
 class ExprLinearizer : public ModelParser {
  public:
-  ExprLinearizer(hash_map<IntVar*, int64>* const variables_to_coefficients)
+  explicit ExprLinearizer(
+      hash_map<IntVar*, int64>* const variables_to_coefficients)
       : variables_to_coefficients_(variables_to_coefficients), constant_(0) {}
 
   ~ExprLinearizer() override {}
@@ -3042,7 +3043,7 @@ IntExpr* MakeSumArrayAux(Solver* const solver, const std::vector<IntVar*>& vars,
   }
 }
 
-IntExpr* MakeSumAux(Solver* const solver, const std::vector<IntVar*> vars,
+IntExpr* MakeSumAux(Solver* const solver, const std::vector<IntVar*>& vars,
                     int64 constant) {
   const int size = vars.size();
   if (size == 0) {

src/constraint_solver/tree_monitor.cc

@@ -432,7 +432,7 @@ class TreeNode {
         tree_writer->AddAttribute("value", "0");
       } else {
         // Use the original size of the first child if available
-        const DomainMap& domain = parent_ && parent_->children_.size()
+        const DomainMap& domain = parent_ && !parent_->children_.empty()
                                       ? parent_->children_[0]->domain()
                                       : domain_;