Problem

Given a tree, rearrange the tree in in-order so that the leftmost node in the tree is now the root of the tree, and every node has no left child and only 1 right child.

Example 1: Input: [5,3,6,2,4,null,8,1,null,null,null,7,9]

         5
        / \
       3    6
     /  \    \
    2    4    8
   /         / \
  1         7   9

Output: [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

 1
  \
   2
    \
     3
      \
       4
        \
         5
          \
           6
            \
             7
              \
               8
                \
                 9

Note:

The number of nodes in the given tree will be between 1 and 100.

Each node will have a unique integer value from 0 to 1000.

Pre analysis

Will do in-order traverse and convert it into array and resultant in tree again

Another Solution

Concise version without additional space

var increasingBST = function(root) {
  return dfs(root);
};

function dfs(node, parent) {
  if (!node) return parent;
  let result = dfs(node.left, node);
  node.right = dfs(node.right, parent);
  node.left = null;
  return result;
}