Added older advents

advent_of_code_2019/day4/main1.cc

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+#include <iostream>
+#include <string>
+
+using namespace std;
+
+bool meets_criteria(size_t passwd)
+{
+    string number = to_string(passwd);
+
+    bool adjacent = false;
+
+    char min = '0';
+    char prev = '1';
+    for (char ch : number)
+    {
+        adjacent = adjacent | (prev == ch);
+        
+        if (ch < min)
+            return false;
+        
+        min = ch;
+        prev = ch;
+    }
+
+    return adjacent;
+}
+
+int main()
+{
+    const size_t LOWER = 178416;
+    const size_t UPPER = 676461;
+
+    size_t valid = 0;
+    for (size_t idx = LOWER; idx <= UPPER; ++idx)
+    {
+        if (meets_criteria(idx))
+            ++valid;
+    }
+
+    cout << "valid: " << valid << "\n";
+}