sicp 1.12

递归过程求解Pascal三角形：

 

(define (pascal-tri row col)
  (cond ((or (< row col) (<= row 0) (<= col 0)) 0)
		((or (= col 1) (= col row)) 1)
		(else (+ (pascal-tri (- row 1) col)
				 (pascal-tri (- row 1) (- col 1))))))