Time: Not done

The main reason I didn’t make it was that I kept thinking about how to use a stack. When I looked at the answer to the stack later, I found that recursion was the simplest.

recursion

The idea is very simple. First, write out how to insert a child node.

  1. If the node is smaller than the parent node and the parent node’s left node is empty, make it the left node.
  2. If the node is larger than the parent node and the parent node’s right node is empty, make it the right node.
  3. If the node is smaller than the parent node, assign the parent node to the parent node’s left node and return to 1.
  4. If the node is larger than the parent node, assign the parent node to the parent node’s right node and return to 1. Traverse the entire array and insert each node one by one to get the result.
var bstFromPreorder = function(preorder) {
    var add = function (node,val) {
        if (val < node.val && !node.left) {
            node.left = new TreeNode(val)
        }
        if (val > node.val && !node.right) {
            node.right = new TreeNode(val)
        }
        if (val < node.val) {
            add(node.left,val)
        }
        if (val > node.val) {
            add(node.right,val)
        }
    }
    var root = new TreeNode(preorder.shift())
    for(let i = 0 ; i < preorder.length ; i ++) {
        add(root,preorder[i])
    }
    return root};

Time: Not done

The main reason I didn’t make it was that I kept thinking about how to use a stack. When I looked at the answer to the stack later, I found that recursion was the simplest.

recursion

The idea is very simple. First, write out how to insert a child node

  1. If the node is smaller than the parent node and the parent node’s left node is empty, make it the left node.
  2. If the node is larger than the parent node and the parent node’s right node is empty, make it the right node.
  3. If the node is smaller than the parent node, assign the parent node to the parent node’s left node and return to 1.
  4. If the node is larger than the parent node, assign the parent node to the parent node’s right node and return to 1. Traverse the entire array and insert each node one by one to get the result.
var bstFromPreorder = function(preorder) {
    var add = function (node,val) {
        if (val < node.val && !node.left) {
            node.left = new TreeNode(val)
        }
        if (val > node.val && !node.right) {
            node.right = new TreeNode(val)
        }
        if (val < node.val) {
            add(node.left,val)
        }
        if (val > node.val) {
            add(node.right,val)
        }
    }
    var root = new TreeNode(preorder.shift())
    for(let i = 0 ; i < preorder.length ; i ++) {
        add(root,preorder[i])
    }
    return root};