Update create-maximum-number.cpp

Author: kamyu104

C++/create-maximum-number.cpp

@@ -1,37 +1,36 @@
 // Time:  O(k * (m + n + k)) ~ O(k * (m + n + k^2))
 // Space: O(m + n + k^2)
 
-// DP + Greedy solution. (48ms)
+// DP + Greedy solution.
 class Solution {
 public:
     vector<int> maxNumber(vector<int>& nums1, vector<int>& nums2, int k) {
-        vector<int> res(k);
         const int m = nums1.size(), n = nums2.size();
-        vector<vector<int>> max_digits1(k + 1), max_digits2(k + 1);
-        getMaxDigits(nums1, max(0, k - n), min(k, m), &max_digits1);  // O(k * m) time, O(m + k^2) space.
-        getMaxDigits(nums2, max(0, k - m), min(k, n), &max_digits2);  // O(k * n) time, O(n + k^2) space.
-        for (int i = max(0, k - n); i <= min(k, m); ++i) {  // k * O(k) ~ k * O(k^2) time.
-            int j = k - i;
+        vector<vector<int>> max_numbers1(k + 1), max_numbers2(k + 1);
+        maxNumberDP(nums1, max(0, k - n), min(k, m), &max_numbers1);  // O(k * m) time, O(m + k^2) space.
+        maxNumberDP(nums2, max(0, k - m), min(k, n), &max_numbers2);  // O(k * n) time, O(n + k^2) space.
+
+        vector<int> res(k);
+        for (int i = max(0, k - n); i <= min(k, m); ++i) {  // k * O(k) ~ k * O(k^2) time
             vector<int> tmp(k);
-            merge(max_digits1[i], max_digits2[j], &tmp);
+            merge(max_numbers1[i], max_numbers2[k - i], &tmp);
             if (tmp > res) {
                 res = move(tmp);
             }
         }
         return res;
     }
-
 private:
-    void getMaxDigits(vector<int> nums, int start, int end, vector<vector<int>> *maxDigits) {
-        (*maxDigits)[end] = maxDigit(nums, end);
+    void maxNumberDP(vector<int> nums, int start, int end, vector<vector<int>> *max_numbers) {
+        (*max_numbers)[end] = maxNumber(nums, end);
         for (int i = end - 1; i >= start; --i) {
-            (*maxDigits)[i] = deleteDigit((*maxDigits)[i + 1]);
+            (*max_numbers)[i] = deleteNumber((*max_numbers)[i + 1]);
         }
     }
 
     // Time:  O(n)
     // Space: O(n)
-    vector<int> maxDigit(const vector<int>& nums, int k) {
+    vector<int> maxNumber(const vector<int>& nums, int k) {
         vector<int> res;
         int drop = nums.size() - k;
         for (const auto& num : nums) {
@@ -47,7 +46,7 @@ class Solution {
 
     // Time:  O(n)
     // Space: O(n)
-    vector<int> deleteDigit(const vector<int>& nums) {
+    vector<int> deleteNumber(const vector<int>& nums) {
         vector<int> res(nums);
         for (int i = 0; i < res.size(); ++i) {
             if (i == res.size() - 1 || res[i] < res[i + 1]) {
@@ -65,30 +64,20 @@ class Solution {
              first2 = vec2.begin(), last2 = vec2.end();
         auto result = res->begin();
         while (first1 != last1 || first2 != last2) {
-            int val1 = first1 != last1 ? *first1 : numeric_limits<int>::min();
-            int val2 = first2 != last2 ? *first2 : numeric_limits<int>::min();
-            if (val1 > val2) {
+            if (greater(first1, last1, first2, last2)) {
                 *result++ = *first1++;
-            } else if (val1 < val2) {
-                *result++ = *first2++;
             } else {
-                auto pos1 = first1 + 1, pos2 = first2 + 1;
-                while (pos1 != last1 || pos2 != last2) {  // O(1) ~ O(k) time.
-                    int val1 = (pos1 != last1) ? *(pos1) : numeric_limits<int>::min();
-                    int val2 = (pos2 != last2) ? *(pos2) : numeric_limits<int>::min();
-                    if (val1 > val2) {
-                        *result++ = *first1++;
-                        break;
-                    } else if (val1 < val2) {
-                        *result++ = *first2++;
-                        break;
-                    }
-                    ++pos1, ++pos2;
-                }
-                if (pos1 == last1 && pos2 == last2) {
-                    *result++ = *first2++;
-                }
+                *result++ = *first2++;
             }
         }
     }
+
+    template<typename IT>
+    bool greater(IT first1, IT last1, IT first2, IT last2) {
+        while (first1 != last1 && first2 != last2 && *first1 == *first2) {
+            ++first1;
+            ++first2;
+        }
+        return (first2 == last2) || (first1 != last1 && *first1 > *first2);
+    }
 };