285. Inorder Successor in BST

Given a binary search tree and a node in it, find the in-order successor of that node in the BST.

The successor of a node p is the node with the smallest key greater than p.val.

Example 1:

Input: root = [2,1,3], p = 1
Output: 2
Explanation: 1's in-order successor node is 2. Note that both p and the return value is of TreeNode type.
  • Iterative

public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
    ArrayDeque<TreeNode> stack = new ArrayDeque<>();
    TreeNode curr = root;
    int prev = Integer.MAX_VALUE;
    while(curr != null || !stack.isEmpty()) {
        while(curr != null) {
            stack.push(curr);
            curr = curr.left;
        }
        curr = stack.pop();
        if(prev == p.val) return curr;
        prev = curr.val;
        curr = curr.right;
    }
    return null;
}
  • Recursive
class Solution {
    TreeNode result;
    public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
        traverse(root, p.val);
        return result;
    }

    public void traverse(TreeNode root, int v) {
        if(root == null) return;
        if(result != null) return;
        traverse(root.left, v);
        if(result == null && root.val > v)
            result = root;
        if(result != null) return;
        traverse(root.right, v);
    }
}