Snapshot Array

Description

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 <= 50000
• At most 50000 calls will be made to set, snap, and get.
• 0 <= index < length
• 0 <= snap_id < (the total number of times we call snap())
• 0 <= val <= 10^9

Solution(javascript)

// /**
//  * @param {number} length
//  */
// const SnapshotArray = function (length) {
//   this.snap_id = 0
//   this.list = new Array(length).fill(0).map(() => ({
//     [this.snap_id]: 0,
//   }))
// }

// /** O(1)
//  * @param {number} index
//  * @param {number} val
//  * @return {void}
//  */
// SnapshotArray.prototype.set = function (index, val) {
//   this.list[index][this.snap_id] = val
// }

// /** O(1)
//  * @return {number}
//  */
// SnapshotArray.prototype.snap = function () {
//   this.snap_id += 1
//   return this.snap_id - 1
// }

// /** 之前的解法是 O(snap_id) 我们可以做的更好, 把它做到 O(log(snapLength)) 或者
//  * @param {number} index
//  * @param {number} snap_id
//  * @return {number}
//  */
// SnapshotArray.prototype.get = function (index, snap_id) {
//   if (this.list[index][snap_id] === undefined) {
//     const keys = Object.keys(this.list[index]).map(key => Number(key)).sort((a, b) => a - b)
//     console.log(keys)
//     for (let i = keys.length - 1; i >= 0; i--) {
//       if (keys[i] <= snap_id) {
//         return this.list[index][keys[i]]
//       }
//     }
//     return 0
//   }
//   return this.list[index][snap_id]
// }
// SnapshotArray.prototype.get = function (index, snap_id) {
//   let current = snap_id
//   while (this.list[index][current] === undefined) {
//     current -= 1
//   }
//   return this.list[index][current]
// }


/** 在 JS 里进一步优化，不初始化
 * @param {number} length
 */
const SnapshotArray = function (length) {
  this.snap_id = 0
  this.list = []
}

/** O(1)
 * @param {number} index
 * @param {number} val
 * @return {void}
 */
SnapshotArray.prototype.set = function (index, val) {
  this.list[index] = this.list[index] || []
  this.list[index][this.snap_id] = val
}

/** O(1)
 * @return {number}
 */
SnapshotArray.prototype.snap = function () {
  this.snap_id += 1
  return this.snap_id - 1
}

/** 之前的解法是 O(snap_id) 我们可以做的更好, 把它做到 O(log(snapLength)) 或者
 * @param {number} index
 * @param {number} snap_id
 * @return {number}
 */
SnapshotArray.prototype.get = function (index, snap_id) {
  if (this.list[index] === undefined) {
    return 0
  }
  if (this.list[index][snap_id] === undefined) {
    const keys = Object.keys(this.list[index]).map(key => Number(key)).sort((a, b) => a - b)
    for (let i = keys.length - 1; i >= 0; i--) {
      if (keys[i] <= snap_id) {
        return this.list[index][keys[i]]
      }
    }
    return 0
  }
  return this.list[index][snap_id]
}