杨辉三角

def triangles():
    prev_line = [1]
    while True:
        yield prev_line
        result = [1]
        if len(prev_line) == 1:
            result.append(1)
        if len(prev_line) > 1:
            for i in range(len(prev_line)-1):
                result.append(prev_line[i]+prev_line[i+1])
            result.append(1)

        prev_line = result