TreeUtil.buildSingle指定rootId节点存在时，作为根节点

hutool-core/src/test/java/org/dromara/hutool/core/tree/IssueIAUSHRTest.java

@@ -0,0 +1,58 @@
+package org.dromara.hutool.core.tree;
+
+import lombok.Data;
+import org.dromara.hutool.core.tree.parser.NodeParser;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class IssueIAUSHRTest {
+	@Test
+	void buildSingleTest() {
+		final List<TestDept> list= new ArrayList<>();
+		TestDept sysDept = new TestDept();
+		sysDept.setDeptId(1L);
+		sysDept.setDeptName("A");
+		sysDept.setParentId(null);
+		list.add(sysDept);
+
+		sysDept = new TestDept();
+		sysDept.setDeptId(2L);
+		sysDept.setDeptName("B");
+		sysDept.setParentId(1L);
+		list.add(sysDept);
+
+		sysDept = new TestDept();
+		sysDept.setDeptId(3L);
+		sysDept.setDeptName("C");
+		sysDept.setParentId(1L);
+		list.add(sysDept);
+
+		final TreeNodeConfig treeNodeConfig = new TreeNodeConfig();
+		treeNodeConfig.setIdKey("deptId");
+		treeNodeConfig.setNameKey("deptName");
+		treeNodeConfig.setParentIdKey("parentId");
+
+		final NodeParser<TestDept,Long> nodeParser= (dept, tree) ->
+			tree.setId(dept.getDeptId())
+				.setParentId(dept.getParentId())
+				.setName(dept.getDeptName());
+
+		final MapTree<Long> longTree = TreeUtil.buildSingle(list, 1L, treeNodeConfig, nodeParser);
+
+		assertEquals("A", longTree.getName());
+		assertEquals(2, longTree.getChildren().size());
+		assertEquals("B", longTree.getChildren().get(0).getName());
+		assertEquals("C", longTree.getChildren().get(1).getName());
+	}
+
+	@Data
+	public static class TestDept {
+		private Long deptId;
+		private String deptName;
+		private Long parentId;
+	}
+}