LeetCode 128 Longest Consecutive Sequence (HashMap)

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.

题目链接：https://leetcode.com/problems/longest-consecutive-sequence/

题目大意：求数值连续的子序列最大长度

题目分析：要求O(n)的时间复杂度，那显然只能HashMap了，向左向右拓展标记即可，再回到复杂度，Java里的HashMap的put和get的复杂度都是O(1)的所以总的复杂度为O(n)，如果用C++的STL的map，其put的复杂度是O(logn)，所以插入的复杂度就是O(nlogn)了，相当于Java里的TreeMap

public class Solution {    public int longestConsecutive(int[] nums) {        HashMap<Integer, Boolean> hash = new HashMap<>();        HashMap<Integer, Boolean> visit = new HashMap<>();        int count = 0, ans = 0;        int len = nums.length;        for (int i = 0; i < len; i++) {            hash.put(nums[i], true);            visit.put(nums[i], false);        }        for (int i = 0; i < len; i++) {            if (!visit.get(nums[i])) {                visit.put(nums[i], true);                int add = 1, sub = -1;                count = 1;                while (hash.containsKey(nums[i] + add)) {                    visit.put(nums[i] + add, true);                    count ++;                    add ++;                }                while (hash.containsKey(nums[i] + sub)) {                    visit.put(nums[i] + sub, true);                    count ++;                    sub --;                }                ans = Math.max(ans, count);            }        }        return ans;    }}