​LeetCode刷题实战128：最长连续序列

示例 1：

输入：nums = [100,4,200,1,3,2]
输出：4
解释：最长数字连续序列是 [1, 2, 3, 4]。它的长度为 4。

示例 2：

输入：nums = [0,3,7,2,5,8,4,6,0,1]
输出：9

class Solution {
    public int longestConsecutive(int[] nums) {
        Set num_set = new HashSet<>();
        for (int n : nums) num_set.add(n);
        int res = 0;
        for (int num : num_set) {
            if (!num_set.contains(num - 1)) {
                int tmp = 1;
                while (num_set.contains(num + 1)) {
                    tmp++;
                    num++;
                }
                res = Math.max(res, tmp);
            }
        }
        return res;
    }
}

class Solution {
    public int longestConsecutive(int[] nums) {
        HashMap lookup = new HashMap<>();
        int res = 0;
        for (int num : nums) {
            if (!lookup.containsKey(num)) {
                // 查看左右两边是否可以相连
                int left = (lookup.containsKey(num - 1)) ? lookup.get(num - 1) : 0;
                int right = (lookup.containsKey(num + 1)) ? lookup.get(num + 1) : 0;
                lookup.put(num, left + right + 1);
                // 改变首尾两个长度(换成更长的长度)
                lookup.put(num - left, left + right + 1);
                lookup.put(num + right, left + right + 1);
                res = Math.max(res, left + right + 1);
            }

        }
        return res;
    }
}