Method buildTree

templates/segment-tree.cpp:26–39

    TreeNode * root;

    TreeNode * buildTree(vector<int>& nums, int l, int r)
    {
        if(l == r){
            // 叶子节点
            return new TreeNode(l, r, nums[l]);
        }

        // 后序构造
        auto left = buildTree(nums, l, (l+r)/2);
        auto right = buildTree(nums, (l+r)/2+1, r);

        // 非叶子节点
        return new TreeNode(l, r, left->v+right->v, left, right);
    }

    void dumpInternal(TreeNode * n, int d){
        if(!n)return;