Python 字符串查找第一个仅出现一次的字符

s = input()
d = {}
for i in s:
    d[i] = d.get(i, 0) + 1
flag = False
for i in s:
    if d[i] == 1:
        print(i)
        flag = True
        break
if not flag:
    print('No')