1146. 快照数组

题目：

思路：

【1】利用TreeMap解题

1）利用TreeMap来存储snap_id : val，然后再用一个Map来存储index : tree；
2）找index索引下的snap_id版本的值时，只需要找最大的小于等于snap_id版本的值，利用TreeMap.floorEntry(snap_id)来实现。

 

代码展示：

//时间69 ms 击败 21.27%
//内存97.7 MB 击败 5.7%
class SnapshotArray {

    // index : (snap_id : val)
    private Map<Integer, TreeMap<Integer, Integer>> map;
    private int snap_id;

    public SnapshotArray(int length) {
        map = new HashMap<>();
        snap_id = 0;
        for (int i = 0; i < length; i++) {
            TreeMap<Integer, Integer> tree = new TreeMap<>();
            tree.put(0, 0);
            map.put(i, tree);
        }
    }

    public void set(int index, int val) {
        TreeMap<Integer, Integer> tree = map.get(index);
        tree.put(snap_id, val);
    }

    public int snap() {
        return snap_id++;
    }

    public int get(int index, int snap_id) {
        TreeMap<Integer, Integer> tree = map.get(index);
        return tree.floorEntry(snap_id).getValue();
    }
}

/**
 * Your SnapshotArray object will be instantiated and called as such:
 * SnapshotArray obj = new SnapshotArray(length);
 * obj.set(index,val);
 * int param_2 = obj.snap();
 * int param_3 = obj.get(index,snap_id);
 */


//简化的数据结果达成效果
//时间48 ms 击败 56.19%
//内存81.2 MB 击败 29.83%
class SnapshotArray {
    int id = 0;
    List<int[]>[] snapshots;

    public SnapshotArray(int length) {
        snapshots = new List[length];
        for (int i = 0; i < length; i++) {
            snapshots[i] = new ArrayList<int[]>();
        }
    }

    public void set(int index, int val) {
        snapshots[index].add(new int[]{id, val});
    }

    public int snap() {
        return id++;
    }

    public int get(int index, int snap_id) {
        List<int[]> snaplist = snapshots[index];
        int low = -1, high = snaplist.size() - 1;
        while (low < high) {
            int mid = low + (high - low + 1) / 2;
            if (snaplist.get(mid)[0] <= snap_id) {
                low = mid;
            } else {
                high = mid - 1;
            }
        }
        return low >= 0 ? snaplist.get(low)[1] : 0;
    }
}