128. Longest Consecutive Sequence

题目128. Longest Consecutive Sequence

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.

1,BFS

public class Solution {
    public int longestConsecutive(int[] nums) {
        if(nums == null || nums.length == 0){
            return 0;
        }
        
        int max = 0;
        Map memory = new HashMap();
        for(int num : nums){
            if(!memory.containsKey(num)){
                int left = memory.containsKey(num - 1) ? memory.get(num-1) : 0;
                int right = memory.containsKey(num + 1) ? memory.get(num+1) : 0;
                
                int sum = left + right + 1;
                memory.put(num,sum);
                max = max > sum ? max : sum;
                
                memory.put(num-left,sum);
                memory.put(num+right,sum);
            }
        }
        
        return max;
    }
}

2,BFS+并查集

public class Solution {
    
    private int[] parents;
    public void initUnionFind(int[] nums){
        if(nums == null){
            return;
        }
        
        parents = new int[nums.length];
        for(int i=0, len=nums.length; i counter[root] ? max : counter[root];
        }
        return max;
    }
    
    
    public int longestConsecutive(int[] nums) {
        if(nums == null || nums.length == 0){
            return 0;
        }
        
        Map memory = new HashMap();
        initUnionFind(nums);
        for(int i=0, len=nums.length; i