Given two binary strings, return their sum (also a binary string).
The input strings are bothnon-emptyand contains only characters1or 0.
Example 1:
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Input: a = "11", b = "1"
Output: "100"
Example 2:
Input: a = "1010", b = "1011"
Output: "10101"
通过前面补0使两个String等长，然后从右往左相加。
