added is_complete_tree in java folder

java/is_complete_tree.java

@@ -0,0 +1,33 @@
+import java.util.LinkedList;
+import java.util.Queue;
+
+public class _958 {
+    public static class Solution1 {
+        public boolean isCompleteTree(TreeNode root) {
+            Queue<TreeNode> queue = new LinkedList<>();
+            queue.offer(root);
+            boolean shouldHaveNoMoreChildren = false;
+            while (!queue.isEmpty()) {
+                int size = queue.size();
+                for (int i = 0; i < size; i++) {
+                    TreeNode curr = queue.poll();
+                    if (shouldHaveNoMoreChildren && (curr.left != null || curr.right != null)) {
+                        return false;
+                    }
+                    if (curr.left == null && curr.right != null) {
+                        return false;
+                    }
+                    if (curr.left != null) {
+                        queue.offer(curr.left);
+                    }
+                    if (curr.right == null) {
+                        shouldHaveNoMoreChildren = true;
+                    } else {
+                        queue.offer(curr.right);
+                    }
+                }
+            }
+            return true;
+        }
+    }
+}