求回文子字符串，如：在ABCDDCBAXz中，DD、CDDC、BCDDCB、ABCDDCBA均为回文子字符串。实现代码如下：

srcStr = input()

symList = [] #保存回文子字符串
for i in range(len(srcStr)):
    for j in range(i + 2, len(srcStr) + 1):
        subStr = ___________
        if subStr == _____________:
            symList.append(subStr)

for i in sorted(symList, key = lambda x: len(x)):
    print(i)