说明：  2.3 主元素问题(课本2-9) 问题描述：设T[0:n-1]是n个元素的数组，如果其中某个元素x在整个数组中的出现次数超过n/2，则称x为数组T的主元素。输入数据由文件名为input.txt的文本文件提供。 请设计一个线性时间算法，判断input中的数据是否存在主元素。在实验报告中对算法时间复杂度作出分析。 输入：文件的第1行为数组S中元素个数n；接下来的n 行中，每行有一个自然数。程序运行结束时，将计算结果输出到文件output.txt中。输出文件中包含问题的答案：找不到主元素时给出null，找到时给出主元素的值。
(2.3 Main elements of the problem (books 2-9) Description of the problem: Let T [0: n-1] is the n-element array, if one of these elements in the array x number of occurrences of more than n/2, called x The main elements of the array T. Input data from a text file named input.txt document. Please design a linear time algorithm to determine whether there is input data from the primary element. In the lab report on an analysis of time complexity. Input: File 1 of act number of elements in the array S in n the next n lines, each line has a natural number. When the run is finished, will result in the output to the file output.txt. Output file contains the answer to the question: Can not find the main element is given null, given the time to find the main element.)