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LongestConsecutiveSequence.java
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LongestConsecutiveSequence.java
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package Leetcode;
import java.util.HashMap;
import java.util.Map;
/**
* @author kalpak
*
* Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence.
*
* Example 1:
* Input: nums = [100,4,200,1,3,2]
* Output: 4
*
* Explanation: The longest consecutive elements sequence is [1, 2, 3, 4].
* Therefore its length is 4.
*
* Example 2:
* Input: nums = [0,3,7,2,5,8,4,6,0,1]
* Output: 9
*
*
* Constraints:
*
* 0 <= nums.length <= 104
* -10^9 <= nums[i] <= 10^9
*
*
* Follow up: Could you implement the O(n) solution?
*
*/
public class LongestConsecutiveSequence {
public static int longestConsecutive(int[] nums) {
DisjointSet dsu = new DisjointSet(nums.length);
Map<Integer, Integer> map = new HashMap<>();
for(int i = 0; i < nums.length; i++) {
if(map.containsKey(nums[i]))
continue;
// number not present in the map.
// Check if nums[i-1] and nums[i+1] is present.
// If so, union them.
// Remember we do the Union Find on indices.
if(map.containsKey(nums[i] - 1))
dsu.union(i, map.get(nums[i] - 1));
if(map.containsKey(nums[i] + 1))
dsu.union(i, map.get(nums[i] + 1));
map.put(nums[i], i);
}
return dsu.getMaximumSizeOfComponent();
}
static private class DisjointSet {
private int[] size; // track size of each component
private int[] id; // id[i] points to the parent of i, if id[i] = i, then i is the root node.
public DisjointSet(int size) {
this.size = new int[size];
this.id = new int[size];
// Initialize the arrays as individual components
for(int i = 0; i < size; i++) {
id[i] = i; // self root
this.size[i] = 1;
}
}
public int find(int p) {
// find the root of the component
int root = p;
while(root != id[root])
root = id[root];
// Path Compression : Gives amortized time complexity
while(p != root) {
int next = id[p]; // Store the id of p and make p point to the root
id[p] = root; // Compress
p = next; // do the same for the rest.
}
return root;
}
public boolean union(int p, int q) {
int root1 = find(p);
int root2 = find(q);
if(root1 == root2)
return false;
// Merge component with smaller size to the component with larger size.
if(size[root1] < size[root2]) {
size[root2] += size[root1];
id[root1] = root2; // Merge
} else {
size[root1] += size[root2];
id[root2] = root1;
}
return true;
}
public int getMaximumSizeOfComponent() {
int max = 0;
for(int i = 0; i < id.length; i++) {
if(id[i] == i && size[i] > max)
max = size[i];
}
return max;
}
}
public static void main(String[] args) {
int[] nums = new int[]{2, 3, 100, 101, 1, 102, 4};
System.out.println(longestConsecutive(nums));
}
}