_128_LongestConsecutiveSequence.java

package leetcode_1To300;

import java.util.HashSet;

/**
 * 本代码来自 Cspiration，由 @Cspiration 提供
 * 题目来源：http://leetcode.com
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 * - 版权所有，转发请注明出处
 */
public class _128_LongestConsecutiveSequence {
    /**
     * 128. 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.

     time : O(n)
     space : O(n)

     * @param nums
     * @return
     */
    public int longestConsecutive(int[] nums) {
        if (nums == null || nums.length == 0) return 0;
        HashSet<Integer> set = new HashSet<>();
        int res = 0;
        for (int num : nums) {
            set.add(num);
        }
        for (int i = 0; i < nums.length; i++) {
            int down = nums[i] - 1;
            while (set.contains(down)) {
                set.remove(down);
                down--;
            }
            int up = nums[i] + 1;
            while (set.contains(up)) {
                set.remove(up);
                up++;
            }
            res = Math.max(res, up - down - 1);
        }
        return res;
    }
}