LeetCode——Longest Consecutive Sequence

pickless

于 2013-09-26 21:38:49 发布

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分类专栏： Online Judge 文章标签： leetcode

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本文探讨了如何使用HashMap在O(n)复杂度内找出给定无序整数数组中最长连续整数序列的长度。

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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.

» Solve this problem

用了HashMap，不能严格地说是O(n)。

public class Solution {
    public int longestConsecutive(int[] num) {
        // Start typing your Java solution below
        // DO NOT write main() function
        HashMap<Integer, Integer> expand = new HashMap<Integer, Integer>();
        for (int i = 0; i < num.length; i++) {
            expand.put(num[i], 1);
        }
        int max = 0;
        for (int i = 0; i < num.length; i++) {
            if (expand.get(num[i]) != -1) {
                int q = 1;
                Integer k = num[i] + 1;
                Integer v = expand.get(num[i] + 1);
                while (v != null) {
                    expand.put(k, -1);
                    q += v;
                    k += v;
                    v = expand.get(k);
                }                
                expand.put(num[i], q);
                if (q > max) {
                    max = q;
                }
            }
        }
        return max;
    }
}