Create array-with-elements-not-equal-to-average-of-neighbors.cpp

Author: kamyu104

C++/array-with-elements-not-equal-to-average-of-neighbors.cpp

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+// Time:  O(n) ~ O(n^2), O(n) on average
+// Space: O(1)
+
+// Tri Partition (aka Dutch National Flag Problem) with virtual index solution
+class Solution {
+public:
+    vector<int> rearrangeArray(vector<int>& nums) {
+        int mid = (size(nums) - 1) / 2;
+        nth_element(begin(nums), begin(nums) + mid, end(nums));  // O(n) ~ O(n^2) time
+        reversedTriPartitionWithVI(nums, nums[mid]);  // O(n) time, O(1) space
+        return nums;
+    }
+
+private:
+    void reversedTriPartitionWithVI(vector<int>& nums, int val) {
+        const int N = size(nums) / 2 * 2 + 1;
+        #define Nums(i) nums[(1 + 2 * (i)) % N]
+        for (int i = 0, j = 0, n = size(nums) - 1; j <= n;) {
+            if (Nums(j) > val) {
+                swap(Nums(i++), Nums(j++));
+            } else if (Nums(j) < val) {
+                swap(Nums(j), Nums(n--));
+            } else {
+                ++j;
+            }
+        }
+    }
+};
+
+// Time:  O(nlogn)
+// Space: O(n)
+// Sorting and reorder solution
+class Solution2 {
+public:
+    vector<int> rearrangeArray(vector<int>& nums) {
+        int mid = (size(nums) - 1) / 2;
+        sort(begin(nums), end(nums));  // O(nlogn) time
+        vector<int> result(size(nums));  // O(n) space
+        for (int i = 0, smallEnd = mid;  i < size(nums); i += 2, --smallEnd) {
+            result[i] = nums[smallEnd];
+        }
+        for (int i = 1, largeEnd = size(nums) - 1; i < size(nums); i += 2, --largeEnd) {
+            result[i] = nums[largeEnd];
+        }
+        return result;
+    }
+};