Count Integers in Intervals
<p>Given an <strong>empty</strong> set of intervals, implement a data structure that can:</p>

<ul>
	<li><strong>Add</strong> an interval to the set of intervals.</li>
	<li><strong>Count</strong> the number of integers that are present in <strong>at least one</strong> interval.</li>
</ul>

<p>Implement the <code>CountIntervals</code> class:</p>

<ul>
	<li><code>CountIntervals()</code> Initializes the object with an empty set of intervals.</li>
	<li><code>void add(int left, int right)</code> Adds the interval <code>[left, right]</code> to the set of intervals.</li>
	<li><code>int count()</code> Returns the number of integers that are present in <strong>at least one</strong> interval.</li>
</ul>

<p><strong>Note</strong> that an interval <code>[left, right]</code> denotes all the integers <code>x</code> where <code>left &lt;= x &lt;= right</code>.</p>

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

<pre>
<strong>Input</strong>
[&quot;CountIntervals&quot;, &quot;add&quot;, &quot;add&quot;, &quot;count&quot;, &quot;add&quot;, &quot;count&quot;]
[[], [2, 3], [7, 10], [], [5, 8], []]
<strong>Output</strong>
[null, null, null, 6, null, 8]

<strong>Explanation</strong>
CountIntervals countIntervals = new CountIntervals(); // initialize the object with an empty set of intervals. 
countIntervals.add(2, 3);  // add [2, 3] to the set of intervals.
countIntervals.add(7, 10); // add [7, 10] to the set of intervals.
countIntervals.count();    // return 6
                           // the integers 2 and 3 are present in the interval [2, 3].
                           // the integers 7, 8, 9, and 10 are present in the interval [7, 10].
countIntervals.add(5, 8);  // add [5, 8] to the set of intervals.
countIntervals.count();    // return 8
                           // the integers 2 and 3 are present in the interval [2, 3].
                           // the integers 5 and 6 are present in the interval [5, 8].
                           // the integers 7 and 8 are present in the intervals [5, 8] and [7, 10].
                           // the integers 9 and 10 are present in the interval [7, 10].
</pre>

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

<ul>
	<li><code>1 &lt;= left &lt;= right &lt;= 10<sup>9</sup></code></li>
	<li>At most <code>10<sup>5</sup></code> calls <strong>in total</strong> will be made to <code>add</code> and <code>count</code>.</li>
	<li>At least <strong>one</strong> call will be made to <code>count</code>.</li>
</ul>
Design; Segment Tree; Ordered Set
class Node {
public:
    Node(int l, int r)
        : l(l)
        , r(r)
        , mid((l + r) / 2)
        , v(0)
        , add(0)
        , left(nullptr)
        , right(nullptr) {}

    int l, r, mid, v, add;
    Node* left;
    Node* right;
};

class SegmentTree {
public:
    SegmentTree()
        : root(new Node(1, 1000000001)) {}

    void modify(int l, int r, int v, Node* node = nullptr) {
        if (node == nullptr) {
            node = root;
        }
        if (l > r) {
            return;
        }
        if (node->l >= l && node->r <= r) {
            node->v = node->r - node->l + 1;
            node->add = v;
            return;
        }
        pushdown(node);
        if (l <= node->mid) {
            modify(l, r, v, node->left);
        }
        if (r > node->mid) {
            modify(l, r, v, node->right);
        }
        pushup(node);
    }

    int query(int l, int r, Node* node = nullptr) {
        if (node == nullptr) {
            node = root;
        }
        if (l > r) {
            return 0;
        }
        if (node->l >= l && node->r <= r) {
            return node->v;
        }
        pushdown(node);
        int v = 0;
        if (l <= node->mid) {
            v += query(l, r, node->left);
        }
        if (r > node->mid) {
            v += query(l, r, node->right);
        }
        return v;
    }

private:
    Node* root;

    void pushup(Node* node) {
        node->v = node->left->v + node->right->v;
    }

    void pushdown(Node* node) {
        if (node->left == nullptr) {
            node->left = new Node(node->l, node->mid);
        }
        if (node->right == nullptr) {
            node->right = new Node(node->mid + 1, node->r);
        }
        if (node->add != 0) {
            Node* left = node->left;
            Node* right = node->right;
            left->add = node->add;
            right->add = node->add;
            left->v = left->r - left->l + 1;
            right->v = right->r - right->l + 1;
            node->add = 0;
        }
    }
};

class CountIntervals {
public:
    CountIntervals() {}

    void add(int left, int right) {
        tree.modify(left, right, 1);
    }

    int count() {
        return tree.query(1, 1000000000);
    }

private:
    SegmentTree tree;
};

/**
 * Your CountIntervals object will be instantiated and called as such:
 * CountIntervals* obj = new CountIntervals();
 * obj->add(left,right);
 * int param_2 = obj->count();
 */