Create smallest-subsequence-of-distinct-characters.cpp

Author: kamyu104

C++/smallest-subsequence-of-distinct-characters.cpp

@@ -0,0 +1,29 @@
+// Time:  O(n)
+// Space: O(1)
+
+class Solution {
+public:
+    string smallestSubsequence(string text) {
+        const int k = 26;
+        vector<int> count(k);
+        for (const auto& c : text) {
+            ++count[c - 'a'];
+        }
+
+        vector<bool> lookup(k);
+        string stk;
+        for (const auto& c : text) {
+            if (!lookup[c - 'a']) {
+                while (!stk.empty() && stk.back() > c &&
+                       count[stk.back() - 'a']) {
+                    lookup[stk.back() - 'a'] = false;
+                    stk.pop_back();
+                }
+                stk.push_back(c);
+                lookup[c - 'a'] = true;
+            }
+            --count[c - 'a'];
+        }
+        return stk; 
+    }
+};