LeetCode刷题实战128：最长连续序列
Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence.
Follow up: Could you implement the O(n) solution?
题意
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
解题
class Solution {
public int longestConsecutive(int[] nums) {
Set<Integer> 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<Integer, Integer> 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;
}
}