AC, why array is ok but vector is error?

Author: eytter48

P703.cpp

@@ -0,0 +1,54 @@
+#include "header.h"
+#include <bits/c++config.h>
+#include <cstddef>
+#include <vector>
+
+class KthLargest {
+public:
+    int knums[20000], nk;
+    int K;
+    KthLargest(int k, vector<int>& nums) {
+        nk = 0;
+        K = k;
+        if (nums.empty()) return;
+        sort(nums.begin(), nums.end(), greater<int>());
+        int n = min<int>(k, nums.size());
+        for (int i=0; i<n ; i++)
+            knums[nk++] = nums[i];
+        // print(knums);
+    }
+    
+    int add(int val) {
+        if (nk >= K && val<=knums[nk-1]) 
+            return knums[nk-1];
+        if (nk <K) 
+            knums[nk++] = val;
+        int i=nk-1;
+        while (i>0 && val>knums[i-1]) {
+            knums[i] = knums[i-1];
+            i--;
+        }
+        knums[i] = val;
+        // print(knums);
+        return knums[K-1];
+    }
+
+};
+
+/**
+ * Your KthLargest object will be instantiated and called as such:
+ * KthLargest* obj = new KthLargest(k, nums);
+ * int param_1 = obj->add(val);
+ */
+
+int main() {
+    int k = 1;
+    vector<int> nums = {};
+    KthLargest *obj = new KthLargest(k, nums);
+    cout << obj->add(-3) << endl;
+    cout << obj->add(-2) << endl;
+    cout << obj->add(-4) << endl;
+    cout << obj->add(0) << endl;
+    cout << obj->add(4) << endl; 
+    return 0;
+}