leetcode_1146

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

Solutions

1. binary search

class SnapshotArray {
public:
    vector<vector<pair<int, int>>> m;
    int snapid = 0;
    SnapshotArray(int length) : m(length, {{0, 0}}) {
    }

    void set(int index, int val) {
        auto & v = m[index];
        if (v.back().first != snapid)
            v.emplace_back(snapid, val);
        else
            v.back().second = val;
    }

    int snap() {
        return snapid++;
    }

    int get(int index, int snap_id) {
        auto & vp = m[index];
        pair<int, int> target = {snap_id, INT_MAX};
        auto find = lower_bound(vp.begin(), vp.end(), target);
        if (find != vp.end() && find->first == snap_id)
            return find->second;
        else
            return prev(find)->second;
    }
};

/**
 * 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);
 */