CREATE TABLE #tbl 
(
 ItemCode CHAR(4),Qty INT,TreeType CHAR(1),Linenum INT,run_total INT
)
INSERT INTO #tbl (itemcode,qty,treetype,linenum)
VALUES('A100', 1,'S', 0),
('A101', 2 , 'I', 1),
('A102', 3 , 'I', 2),
('B100', 3 , 'S', 3),
('B101', 2 , 'I', 4),
('B102', 2 , 'I', 5),
('C100', 5, 'N', 6)

CREATE CLUSTERED INDEX IX_#tbl_Linenum
ON #tbl (Linenum DESC);

DECLARE 
    @Sequence INT = 0,
	@runtotal INT = 0,
	@grpchange CHAR(1) =  ' '
	
;WITH safetycheck AS
(
	SELECT 
		itemcode,
		qty,
		treetype,
		run_total,
        SEQUENCE = ROW_NUMBER() OVER (ORDER BY linenum DESC)
	FROM #tbl
)
UPDATE t
   SET @Sequence = CASE WHEN SEQUENCE = @Sequence + 1 THEN @Sequence + 1
                   ELSE 1/0 END,
       @runtotal 
			= run_total 
			= CASE 
			    WHEN @grpchange = LEFT(ItemCode,1) 
				THEN @runtotal + 
				 CASE 
				  WHEN treetype ='S' 
				  THEN 0 ELSE qty 
				 END 
				ELSE qty 
				END,
       @grpchange = LEFT(ItemCode,1) -- ANCHOR COLUMN
  FROM SafetyCheck t WITH (TABLOCKX)
OPTION (MAXDOP 1);

SELECT itemcode,run_total,treetype,linenum 
FROM #tbl
WHERE STUFF(itemcode,1,1,'')='100'
ORDER BY linenum;