Add day 1 solution

01/doit.cc

@@ -0,0 +1,57 @@
+// -*- C++ -*-
+// g++ -std=c++20 -Wall -g -o doit doit.cc
+// ./doit 1 < input  # part 1
+// ./doit 2 < input  # part 2
+
+#include <iostream>
+#include <cassert>
+#include <vector>
+#include <algorithm>
+
+using namespace std;
+
+void part1(vector<int> const &l1, vector<int> const &l2) {
+  assert(l1.size() == l2.size());
+  int result = 0;
+  for (size_t i = 0; i < l1.size(); ++i)
+    result += abs(l1[i] - l2[i]);
+  cout << result << '\n';
+}
+
+void part2(vector<int> const &l1, vector<int> const &l2) {
+  // Advance past maximal range matching n, return length of range
+  auto advance = [](int n, size_t &i, vector<int> const &l) {
+    while (i < l.size() && l[i] < n)
+      ++i;
+    auto start = i;
+    while (i < l.size() && l[i] == n)
+      ++i;
+    return i - start;
+  };
+  int result = 0;
+  for (size_t i1 = 0, i2 = 0; i1 < l1.size(); ) {
+    auto n = l1[i1];
+    result += n * advance(n, i1, l1) * advance(n, i2, l2);
+  }
+  cout << result << '\n';
+}
+
+int main(int argc, char **argv) {
+  if (argc != 2) {
+    cerr << "usage: " << argv[0] << " partnum < input\n";
+    exit(1);
+  }
+  vector<int> l1, l2;
+  int n1, n2;
+  while (cin >> n1 >> n2) {
+    l1.push_back(n1);
+    l2.push_back(n2);
+  }
+  sort(l1.begin(), l1.end());
+  sort(l2.begin(), l2.end());
+  if (*argv[1] == '1')
+    part1(l1, l2);
+  else
+    part2(l1, l2);
+  return 0;
+}

01/input1

@@ -0,0 +1,6 @@
+3   4
+4   3
+2   5
+1   3
+3   9
+3   3