Create increasing-decreasing-string.cpp

Author: kamyu104

C++/increasing-decreasing-string.cpp

@@ -0,0 +1,75 @@
+// Time:  O(n)
+// Space: O(1)
+
+class Solution {
+public:
+    string sortString(string s) {
+        string result;
+        auto count = counter(s);
+        while(result.length() != s.length()) {
+            for (int c = 0; c < count.size(); ++c) {
+                if (count[c]) {
+                    result += ('a' + c);
+                    --count[c];
+                }
+            }
+            for (int c = count.size() - 1; c >= 0; --c) {
+                if (count[c]) {
+                    result += ('a' + c);
+                    --count[c];
+                }
+            }
+        }
+        return result;
+    }
+
+private:
+    vector<int> counter(const string& word) {
+        vector<int> count(26);
+        for (const auto& c : word) {
+            ++count[c - 'a'];
+        }
+        return count;
+    }
+};
+
+
+// Time:  O(n)
+// Space: O(1)
+class Solution2 {
+public:
+    string sortString(string s) {
+        string result;
+        for (auto [count, desc] = tuple{counter(s), false}; !count.empty(); desc = !desc) {
+            if (!desc) {
+                for (auto it = begin(count); it != end(count);) {
+                    result.push_back(it->first);
+                    if (!--it->second) {
+                        it = count.erase(it);
+                    } else {
+                        ++it;
+                    }
+                }
+            } else {
+                for (auto rit = rbegin(count); rit != rend(count);) {
+                    result.push_back(rit->first);
+                    if (!--rit->second) {
+                        rit = reverse_iterator(count.erase(next(rit).base()));
+                    } else {
+                        ++rit;
+                    }
+                }
+            }
+        }
+        return result;
+    }
+
+private:
+    map<int, int> counter(const string& word) {
+        map<int, int> count;
+        for (const auto& c : word) {
+            ++count[c];
+        }
+        return count;
+    }
+};