Snapshot Array
<p>Implement a SnapshotArray that supports the following interface:</p>

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

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> [&quot;SnapshotArray&quot;,&quot;set&quot;,&quot;snap&quot;,&quot;set&quot;,&quot;get&quot;]
[[3],[0,5],[],[0,6],[0,0]]
<strong>Output:</strong> [null,null,0,null,5]
<strong>Explanation: </strong>
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</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= length &lt;= 5 * 10<sup>4</sup></code></li>
	<li><code>0 &lt;= index &lt; length</code></li>
	<li><code>0 &lt;= val &lt;= 10<sup>9</sup></code></li>
	<li><code>0 &lt;= snap_id &lt; </code>(the total number of times we call <code>snap()</code>)</li>
	<li>At most <code>5 * 10<sup>4</sup></code> calls will be made to <code>set</code>, <code>snap</code>, and <code>get</code>.</li>
</ul>
Design; Array; Hash Table; Binary Search
class SnapshotArray {
public:
    SnapshotArray(int length) {
        arr.resize(length);
    }

    void set(int index, int val) {
        arr[index].emplace_back(i, val);
    }

    int snap() {
        return i++;
    }

    int get(int index, int snap_id) {
        auto it = upper_bound(arr[index].begin(), arr[index].end(), make_pair(snap_id, INT_MAX));
        return it == arr[index].begin() ? 0 : prev(it)->second;
    }

private:
    vector<vector<pair<int, int>>> arr;
    int i = 0;
};

/**
 * 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);
 */