[LeetCode] 1146. Snapshot Array

Implement a SnapshotArray that supports the following interface:
SnapshotArray(int length) initializes an array-like data structure with the given length. Initially, each element equals 0.
void set(index, val) sets the element at the given index to be equal to val.
int snap() takes a snapshot of the array and returns the snap_id: the total number of times we called snap() minus 1.
int get(index, snap_id) returns the value at the given index, at the time we took the snapshot with the given snap_id

Example 1:
Input: [“SnapshotArray”,”set”,”snap”,”set”,”get”]
[[3],[0,5],[],[0,6],[0,0]]
Output: [null,null,0,null,5]
Explanation:
SnapshotArray snapshotArr = new SnapshotArray(3); // set the length to be 3
snapshotArr.set(0,5); // Set array[0] = 5
snapshotArr.snap(); // Take a snapshot, return snap_id = 0
snapshotArr.set(0,6);
snapshotArr.get(0,0); // Get the value of array[0] with snap_id = 0, return 5

Constraints:
1 <= length <= 5 * 104
0 <= index < length
0 <= val <= 109
0 <= snap_id < (the total number of times we call snap())
At most 5 * 104 calls will be made to set, snap, and get.

快照数组。

实现支持下列接口的「快照数组」- SnapshotArray：

SnapshotArray(int length) - 初始化一个与指定长度相等的 类数组 的数据结构。初始时，每个元素都等于 0。

void set(index, val) - 会将指定索引 index 处的元素设置为 val。

int snap() - 获取该数组的快照，并返回快照的编号 snap_id（快照号是调用 snap() 的总次数减去 1）。

int get(index, snap_id) - 根据指定的 snap_id 选择快照，并返回该快照指定索引 index 的值。

思路

创建一个 hashmap <Integer, TreeMap<Integer, Integer>> map，其中 hashmap 的 key 是数组的下标 index，treemap 里存的是<timestamp, value>。

复杂度

时间：初始化O(n), set() - O(logn), get() - O(1), 整体O(n)
空间O(n)

代码

Java实现

class SnapshotArray {
    int snapId;
    // <index, <snapId, value>>
    HashMap<Integer, TreeMap<Integer, Integer>> map;

    public SnapshotArray(int length) {
        map = new HashMap<>();
        for (int i = 0; i < length; i++) {
            map.put(i, new TreeMap<>());
            map.get(i).put(0, 0);
        }
        snapId = 0;
    }
    
    public void set(int index, int val) {
        TreeMap<Integer, Integer> tmap = map.get(index);
        if (tmap == null) {
            tmap = new TreeMap<>();
            tmap.put(snapId, val);
            map.put(index, tmap);
        } else {
            map.get(index).put(snapId, val);
        }
    }
    
    public int snap() {
        return snapId++;
    }
    
    public int get(int index, int snap_id) {
        return map.get(index).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);
 */