Update finding-3-digit-even-numbers.cpp

Author: kamyu104

C++/finding-3-digit-even-numbers.cpp

@@ -1,7 +1,45 @@
-// Time:  O(n), n is 10^3
+// Time:  O(1) ~ O(n), n is 10^3
 // Space: O(1)
 
 class Solution {
+public:
+    vector<int> findEvenNumbers(vector<int>& digits) {
+        vector<int> cnt(10);
+        for (const auto& d : digits) {
+            ++cnt[d];
+        }
+        vector<int> result, curr;
+        backtracking(&curr, &cnt, &result);
+        return result;
+    }
+
+private:
+    void backtracking(vector<int> *curr, vector<int> *cnt, vector<int> *result) {
+        static const int k = 3;
+        if (size(*curr) == k) {
+            result->emplace_back(accumulate(cbegin(*curr), cend(*curr), 0,
+                                            [](const auto& total, const auto& x) {
+                                                return total * 10 + x;
+                                            }));
+            return;
+        }
+        for (int i = 0; i < size(*cnt); ++i) {
+            if ((*cnt)[i] == 0 || (empty(*curr) && (i == 0)) ||
+                (size(*curr) == k - 1 && i % 2 != 0)) {
+                continue;
+            }
+            --(*cnt)[i];
+            curr->emplace_back(i);
+            backtracking(curr, cnt, result);
+            curr->pop_back();
+            ++(*cnt)[i];
+        }
+    }
+};
+
+// Time:  O(n), n is 10^3
+// Space: O(1)
+class Solution2 {
 public:
     vector<int> findEvenNumbers(vector<int>& digits) {
         unordered_map<int, int> cnt;
@@ -24,7 +62,7 @@ class Solution {
 
 // Time:  O(1) ~ O(n), n is 10^3
 // Space: O(1)
-class Solution2 {
+class Solution3 {
 private:
     struct Node {
         int digit, cnt, left, right;
@@ -90,14 +128,14 @@ class Solution2 {
                     (*digit_cnt_list)[(*digit_cnt_list)[i].right].left = i;
                 }
             }
-            ++((*digit_cnt_list)[i]).cnt;
+            ++(*digit_cnt_list)[i].cnt;
         }
     }
 };
 
 // Time:  O(1) ~ O(nlogn), n is 10^3
 // Space: O(1)
-class Solution3 {
+class Solution4 {
 public:
     vector<int> findEvenNumbers(vector<int>& digits) {
         unordered_map<int, int> cnt;
@@ -142,7 +180,7 @@ class Solution3 {
                 digit_cnt->emplace_back(digit, cnt);
             }
             swap((*digit_cnt)[i], (*digit_cnt).back());
-            ++((*digit_cnt)[i]).second;
+            ++(*digit_cnt)[i].second;
         }
     }
 };