Easier version:

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class Solution {
    public int minMeetingRooms(int[][] intervals) {
        if (intervals == null || intervals.length == 0) return 0;
        int n = intervals.length;
        int[] start = new int[n];
        int[] end = new int[n];
        for (int i = 0; i < n; ++i) {
            start[i] = intervals[i][0];
            end[i] = intervals[i][1];
        }
        Arrays.sort(start);
        Arrays.sort(end);
        
        int res = 0, endIndex =0;
        for (int i = 0; i < n; ++i) {
            if (start[i] < end[endIndex]) res++;
            else endIndex++;
        }
        return res;
    }
}

Previous PriorityQueue version:

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class Solution {
    public int minMeetingRooms(int[][] intervals) {
        if (intervals == null || intervals.length == 0) return 0;
        Arrays.sort(intervals, new Comparator<int[]>(){
            public int compare(int[] a, int[] b) {
                return a[0] - b[0];
            }
        });
        
        PriorityQueue<int[]> pq = new PriorityQueue(intervals.length, new Comparator<int[]>(){
            public int compare(int[] a, int[] b) {
                return a[1] - b[1];
            }
        });
        
        pq.offer(intervals[0]);
        for (int i = 1; i < intervals.length; ++i) {
            int[] interval = pq.poll();
            if (interval[1] <= intervals[i][0]) {
                interval[1] = intervals[i][1];
                pq.offer(interval);
            } else {
                pq.offer(intervals[i]);
                pq.offer(interval);
            }
            
        }
        
        return pq.size();
    }
}