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test18.java
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package byteDance;
import java.util.HashMap;
/**
* Created by lizeyang on 2020/5/13.
* 给定一个未排序的整数数组，找出最长连续序列的长度。
* 要求算法的时间复杂度为 O(n)。
*/
public class test18 {
//时间复杂度O(n),就要牺牲空间复杂度了，使用HashMap
public static int test(int[] nums) {
if (nums == null || nums.length == 0) {
return 0;
}
HashMap<Integer, Boolean> map = new HashMap<>();
for (int num : nums) {
map.put(num, false);
}
int maxLen = 0;
for (int num : nums) {
if (!map.get(num)) {
maxLen = Math.max(maxLen, getMaxLen(map, num));
}
}
return maxLen;
}
//以当前数向前和向后依次判断
public static int getMaxLen(HashMap<Integer, Boolean> map, int num) {
if (map.containsKey(num) && !map.get(num)) {
map.put(num, true);
return 1 + getMaxLen(map, num - 1) + getMaxLen(map, num + 1);
} else {
return 0;
}
}
public static void main(String[] args) {
int[] nums = {4, 200, 3, 1, 100, 2, 99, 101, 5};
System.out.println(test(nums));
}
}