求数组中包含的最长子序列

问题描述：

Given an int array which might contain duplicates, find the largest subset of it which form a sequence.
Eg. {1,6,10,4,7,9,5}
then ans is 4,5,6,7

解答：线性时间，空间为常数（该数组中最大元素的值）

算法：

public int[] longestConsecutiveSequence(int[] a)
        {
                int first = Integer.MAX_VALUE; // the first number of the maximum consecutive sequence
                int length = 0; // the length of the maximum consecutive sequence
                Map<Integer, Integer> table = new HashMap<Integer, Integer>();
                for(int i: a) {
                        if(!table.containsKey(i)) {
                                int start = i;
                                int end = i;
                                if(table.containsKey(i + 1) && table.get(i + 1) >= i + 1) {
                                        end = table.get(i + 1);
                                        table.remove(i + 1);
                                        table.remove(end);
                                }
                                if(table.containsKey(i - 1) && table.get(i - 1) <= i - 1) {
                                        start = table.get(i - 1);
                                        table.remove(i - 1);
                                        table.remove(start);
                                }
                                table.put(start, end);
                                table.put(end, start);
                                if(end - start + 1 > length) {
                                        first = start;
                                        length = end - start + 1;
                                }
                        }
                }
                System.out.println(table);
                System.out.println(length);
                int[] s = new int[length];
                for(int i = 0; i < length; i++) s[i] = first + i;
                return s;
        }