[刷题]Longest Consecutive Sequence

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[LintCode]Longest Consecutive Sequence

Version 1 暴力方法时间复杂度高

public class Solution {    /**     * @param nums: A list of integers     * @return an integer     */    public int longestConsecutive(int[] num) {        // 2015-09-03 暴力方法 O(nlogn)         // num可能含重复元素        if (num == null || num.length == 0) {            return -1;        }        Arrays.sort(num); // O(nlogn)        // 去掉重复元素        ArrayList<Integer> list = new ArrayList<>();        for (int i = 0; i < num.length; i++) {            if (i != 0 && num[i - 1] == num[i]) {                continue;            }            list.add(num[i]);        }                int max = 0;        for (int i = 0; i < list.size(); i++) {            if (i != 0 && list.get(i) == list.get(i - 1) + 1) {                continue;            }            int sum = 1;            int index = i - 1;            int val = list.get(i);            while (index >= 0 && list.get(index) == val - 1) {                sum++;                index--;                val--;            }            index = i + 1;            val = list.get(i);            while (index < list.size() && list.get(index) == val + 1) {                sum++;                index++;                val++;            }            max = max < sum ? sum : max;        }        return max;    }}

Version 2 用HashMap降低时间复杂度

public class Solution {    /**     * @param nums: A list of integers     * @return an integer     */    public int longestConsecutive(int[] num) {        // write you code here        // HashMap: space O(n), time O(n)        // 用hashmap降低时间复杂度        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();        for(int i: num) {            map.put(i, 0);        }                int rst = 1;        // 遍历 用hashmap做记录，已经遍历的为1，没有遍历的为0        for(int key: num) {            if (map.get(key) == 1) continue;            int max = 1;                        int tmp = key;            while(map.containsKey(tmp+1)) {                max++;                tmp++;                map.put(tmp, 1);// 1是一个标志，表示这个点已经被算过了            }            tmp = key;            while(map.containsKey(tmp-1)) {                max++;                tmp--;                map.put(tmp, 1);            }                        map.put(key, 1);                rst = Math.max(max, rst);        }        return rst;    }}

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