运行了以下就很清楚了
a = "aaabbbccc"
p = ""
b = ""
print("p = p + i")
for i in a :
p = p + i
print("p["+i+"]",p)
print("b = i + b")
for i in a:
b = i + b
print("b[" + i + "]", b)
print(p,"\n",b)
输出结果:
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/d4628535e5dde7114322de87a9efce1b9c1661cb?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
p = p + i: 字符串的每一位顺序拼接,所以序列不变;
b = i + b: 字符串的每一位添加在上一位的前面,实现反序输出
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