问题：字符串过滤，仅保留字符串第一次出现的字符。 方法：首先遍历字符串，把第一次出现的字符存入输出字符数组中，输出字符指针递增，以保存下一个；第二次出现的字符不存入输出字符数组中；以此类推，最终得到结果。-Problem: string filter, retaining only the string first characters. Method: First traversal string, the first occurrence of the character stored in the output array of characters, the output character pointer increments, to save the next　second character is not stored in the output array of characters　and so on, and ultimately results.