剑指 Offer II 052. 展平二叉搜索树剑指 Offer II 052. 展平二叉搜索树二叉搜索树，中序遍历，

二叉搜索树，中序遍历，得到有序数组，再循环创建节点拼接起来

var increasingBST = function (root) {
  var res = [];
  var f = (node) => {
    if (!node) {
      return;
    }
    f(node.left);
    res.push(node.val);
    f(node.right);
  };
  f(root);
  var node = new TreeNode(-1);
  var currNode = node;
  for (var value of res) {
    currNode.right = new TreeNode(value);
    currNode = currNode.right;
  }
  return node.right;
};