230. Kth-Smallest-Element-in-a-BST

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} k
 * @return {number}
 */
var kthSmallest = function(root, k) {
    res = 0
    function helper(root){
        if(!root)
            return;
        helper(root.left)
        k--;
        if(k == 0){
            res = root.val;
            return;
        }
        helper(root.right)
    }
    helper(root)
    
    return res
};


# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
        res = 0
        def helper(root):
            nonlocal k
            nonlocal res
            if not root:
                return
            helper(root.left)
            k -= 1
            if k == 0:
                res = root.val
                return
            helper(root.right)

        helper(root)
        return res


# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
        stack = []

        while root or stack:
            if root:
                stack.append(root)
                root = root.left
            else:
                root = stack.pop()
                k -= 1
                if k == 0:
                    return root.val
                root = root.right