leetcode — longest-consecutive-sequence

import java.util.HashSet;
import java.util.Set;

/**
 * Source : https://oj.leetcode.com/problems/longest-consecutive-sequence/
 *
 *
 * Given an unsorted array of integers, find the length of the longest consecutive elements sequence.
 *
 * For example,
 * Given [100, 4, 200, 1, 3, 2],
 * The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.
 *
 * Your algorithm should run in O(n) complexity.
 */
public class LongestConsecutiveSequence {

    /**
     * 求出数组中元素能构成连续序列的最大长度
     *
     * 题目中要求是O(n)，所以不能使用排序的办法
     * 针对每个元素arr[i]寻找数组中向前、向后有没有能构成连续序列的元素，查找的时候利用hash表
     * 每次记录最大长度，如果找到了就把该元素从hash表中删除，因为该元素不会被再次查找到，所以从hash表中移除
     * 如果hash表为空了，说明所有元素都被查找过，退出循环
     *
     * @param arr
     * @return
     */
    public int findLongestSequence (int[] arr) {
        int max = 0;
        Set<Integer> set = new HashSet<Integer>(arr.length);
        for (int i = 0; i < arr.length; i++) {
            set.add(arr[i]);
        }

        for (int i = 0; i < arr.length; i++) {
            int curnum = arr[i] - 1;
            int len = 1;

            // 查找curnum左边的数字
            while (set.contains(curnum)) {
                set.remove(curnum);
                curnum -= 1;
                len++;
            }

            curnum = arr[i] + 1;
            while (set.contains(curnum)) {
                set.remove(curnum);
                curnum += 1;
                len++;
            }

            max = Math.max(max, len);
            if (set.size() <= 0) {
                return max;
            }
        }
        return max;
    }

    public static void main(String[] args) {
        LongestConsecutiveSequence longestConsecutiveSequence = new LongestConsecutiveSequence();
        System.out.println(longestConsecutiveSequence.findLongestSequence(new int[]{100, 4, 200, 1, 3, 2}) + "----4");
    }
}