2020 day1

1 files changed, 17 insertions, 7 deletions

diff --git a/src/2020/day1/aoc.cpp b/src/2020/day1/aoc.cpp
index ab8e2f4..9332c79 100644
--- a/src/2020/day1/aoc.cpp
+++ b/src/2020/day1/aoc.cpp
@@ -1,5 +1,6 @@
 #include "aoc.h"
 #include <set>
+#include <utility>
 #include <vector>
 
 namespace aoc2020 {
@@ -13,20 +14,29 @@ int get_number(const char* p) {
   return d;
 }
 
-int two_sum(const std::vector<int>& is, int target) {
+int two_sum(int* is, size_t s, int target) {
   std::set<int> pairs;
-  for (auto i : is) {
-    auto it = pairs.find(i);
+  for (size_t i = 0; i < s; i++) {
+    auto it = pairs.find(is[i]);
     if (it == pairs.end()) {
-      pairs.insert(target - i);
+      pairs.insert(target - is[i]);
     } else {
-      return i * (target - *it);
+      return is[i] * (target - *it);
     }
   };
   return 0;
 }
 
-int three_sum(const std::vector<int>& is, int target) { return 0; }
+int three_sum(std::vector<int>& is, int target) {
+  for (size_t i = 0; i < is.size(); i++) {
+    std::swap(is[0], is[i]);
+    int x = two_sum(&is[1], is.size() - 1, target - is[0]);
+    if (x > 0) {
+      return is[0] * x;
+    }
+  }
+  return 0;
+}
 
 std::pair<int, int> day1(line_view file, int target) {
   std::vector<int> is;
@@ -34,7 +44,7 @@ std::pair<int, int> day1(line_view file, int target) {
     is.emplace_back(get_number(lv.line));
     return true;
   });
-  return {two_sum(is, target), three_sum(is, target)};
+  return {two_sum(is.data(), is.size(), target), three_sum(is, target)};
 }
 
 } // namespace aoc2020