feat(leetcode): add No.2784

Author: mickey0524

2784.Check-if-Array-is-Good.java

@@ -0,0 +1,34 @@
+// https://leetcode.com/problems/check-if-array-is-good/
+// algorithms
+// Easy (48.73%)
+// Total Accepted: 21.1K
+// Total Submissions: 43.3K
+
+
+class Solution {
+
+    public boolean isGood(int[] nums) {
+        int len = nums.length;
+
+        int[] flag = new int[len + 1];
+        for (int n : nums) {
+            if (n >= len) {
+                return false;
+            }
+
+            flag[n] += 1;
+            if (flag[n] > 1 && n != len - 1) {
+                return false;
+            }
+        }
+
+        for (int i = 1; i < len; i++) {
+            if (flag[i] == 0) {
+                return false;
+            }
+        }
+
+        return flag[len - 1] == 2;
+    }
+
+}