[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);
*/