[leetcode]Longest Consecutive Sequence

这道题目一看就去翻答案了，因为以前见过，知道并查集可以用来做分组，但这样的题目真的要并查集么？想想也是HashSet之类就能搞定么。果然是HashMap而不是HashSet。下面这个解法我比较容易理解，刚开始乍看发现对每一个n又要左边遍历右边遍历的，仔细一看对每一段遍历过的数字就标记一下，那么保证每个数字只访问一次，还是O(n)。

public class Solution {public int longestConsecutive(int[] num) {HashMap<Integer, Boolean> map = new HashMap<Integer, Boolean>();int max = 0;for (int n : num){map.put(n, false);}for (int n: num){if (!map.get(n)){int len1 = getLength(map, n-1, -1);int len2 = getLength(map, n+1, 1);int len = len1 + len2 + 1;if (len > max) max = len;}}return max;}private int getLength(HashMap<Integer, Boolean> map, int n, int step){int len = 0;while (map.containsKey(n)){len++;map.put(n, true);n += step;}return len;}
}

转载于:https://www.cnblogs.com/lautsie/p/3315900.html

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