Added older advents

advent_of_code_2018/day5/main.cc

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+#include <algorithm>
+#include <iostream>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+size_t collapse_polymer(vector<char> polymer)
+{
+    int react_diff = abs('a' - 'A');
+    size_t oldsize = 0;
+    while(oldsize != polymer.size())
+    {
+        oldsize = polymer.size();
+
+        for (size_t idx = 0; idx < polymer.size() - 1; ++idx)
+        {
+            if (abs(polymer[idx] - polymer[idx + 1]) == react_diff)
+            {
+                polymer.erase(polymer.begin() + idx, polymer.begin() + idx + 2);
+                --idx;
+            }
+        }
+    }
+
+    return polymer.size();
+}
+
+int main()
+{
+    
+    vector<char> polymer {
+        istreambuf_iterator<char>(cin),
+        istreambuf_iterator<char>()
+    };
+    cout << "Read polymer of size: " << polymer.size() << "\n";
+
+    size_t minsize = polymer.size();
+    for (size_t idx = 0; idx < 26; ++idx)
+    {
+        char current = 'a' + idx;
+        vector<char> filtered{polymer};
+
+        auto it = remove_if(filtered.begin(), filtered.end(), [=](char c)
+        {
+            if (c == current || c == toupper(current))
+                return true;
+            return false;
+        });
+        filtered.erase(it, filtered.end());
+
+        minsize = min(collapse_polymer(filtered), minsize);
+    }
+
+    cout << "Smallest collapsed polymer size: " << minsize << "\n";
+}