Test55.java

/**
 * Author: 王俊超
 * Date: 2015-06-15
 * Time: 14:54
 * Declaration: All Rights Reserved !!!
 */
public class Test55 {
    /**
     * 题目：请实现一个函数用来找出字符流中第一个只出现一次的字符。
     */
    private static class CharStatistics {
        // 出现一次的标识
        private int index = 0;
        private int[] occurrence = new int[256];

        public CharStatistics() {
            for (int i = 0; i < occurrence.length; i++) {
                occurrence[i] = -1;
            }
        }

        private void insert(char ch) {
            if (ch > 255) {
                throw new IllegalArgumentException( ch + "must be a ASCII char");
            }

            // 只出现一次
            if (occurrence[ch] == -1) {
                occurrence[ch] = index;
            } else {
                // 出现了两次
                occurrence[ch] = -2;
            }

            index++;
        }

        public char firstAppearingOnce(String data) {
            if (data == null) {
                throw new IllegalArgumentException(data);
            }

            for (int i = 0; i < data.length(); i++) {
                insert(data.charAt(i));
            }
            char ch = '\0';
            // 用于记录最小的索引，对应的就是第一个不重复的数字
            int minIndex = Integer.MAX_VALUE;
            for (int i = 0; i < occurrence.length; i++) {
                if (occurrence[i] >= 0 && occurrence[i] < minIndex) {
                    ch = (char) i;
                    minIndex = occurrence[i];
                }
            }

            return ch;
        }
    }

    public static void main(String[] args) {
        System.out.println(new CharStatistics().firstAppearingOnce("")); // '\0'
        System.out.println(new CharStatistics().firstAppearingOnce("g")); // 'g'
        System.out.println(new CharStatistics().firstAppearingOnce("go")); // 'g'
        System.out.println(new CharStatistics().firstAppearingOnce("goo")); // 'g'
        System.out.println(new CharStatistics().firstAppearingOnce("goog")); // '\0'
        System.out.println(new CharStatistics().firstAppearingOnce("googl")); // l
        System.out.println(new CharStatistics().firstAppearingOnce("google")); // l
    }
}