Update Sudoku Solver to check for naked singles.

solver/puzzle.cpp

@@ -1,5 +1,6 @@
 #include "puzzle.h"
 
+#include <algorithm>
 #include <cassert>
 #include <iostream>
 #include <sstream>
@@ -63,6 +64,30 @@ std::string Puzzle::PencilMarkState() {
   return temp;
 }
 
+bool Puzzle::IsSolved() {
+  return std::all_of(cells_.begin(), cells_.end(),
+                     [](const Cell& c) { return c.IsSolved(); });
+}
+
+bool Puzzle::ApplyNextStep() {
+  // Search for a naked single.
+  for (int i = 0; i < 81; i++) {
+    if (cells_[i].IsSolved()) {
+      continue;
+    }
+
+    if (cells_[i].NumPossibilities() == 1) {
+      for (uint8_t v = 1; v <= 9; v++) {
+        if (cells_[i].IsPossible(v)) {
+          AssignSquare(i, v);
+        }
+      }
+      return true;
+    }
+  }
+  return false;
+}
+
 void Puzzle::AssignSquare(uint8_t id, uint8_t value) {
   assert(id < 81);
   assert(value >= 0 && value <= 9);