cpp: add «387. First Unique Character in a String»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cpp/first-unique-character-in-a-string.cpp

@@ -0,0 +1,44 @@
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <string>
+#include <utility>
+
+class Solution {
+  public:
+    int firstUniqChar(const std::string &s) {
+        std::array<std::pair<int, int>, 26> counters;
+        counters.fill(std::make_pair(0, static_cast<int>(s.size())));
+
+        for (auto i = 0; i < static_cast<int>(s.size()); ++i) {
+            auto &[count, index] = counters[s[i] - 'a'];
+
+            ++count;
+            index = std::min(index, i);
+        }
+
+        int min_index = static_cast<int>(s.size());
+        for (const auto &[count, index] : counters) {
+            if (count != 1) {
+                continue;
+            }
+
+            min_index = std::min(min_index, index);
+        }
+
+        if (min_index == static_cast<int>(s.size())) {
+            return -1;
+        }
+        return min_index;
+    }
+};
+
+int main() {
+    Solution s;
+
+    assert(s.firstUniqChar("leetcode") == 0);
+    assert(s.firstUniqChar("loveleetcode") == 2);
+    assert(s.firstUniqChar("aabb") == -1);
+
+    return 0;
+}