235. Lowest Common Ancestor of a Binary Search Tree[Medium]

235. Lowest Common Ancestor of a Binary Search Tree

Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Constraints:

• The number of nodes in the tree is in the range [2, 105].
• -10^9 <= Node.val <= 10^9
• All Node.val are unique.
• p != q
• p and q will exist in the BST.

Example

Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
Output: 6
Explanation: The LCA of nodes 2 and 8 is 6.

题解

    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        // BST特性 左子树小于当前节点，右子树大于当前节点
        // 所以如果要找的两个节点都小于当前节点，那就去左子树里继续找
        if (p.val < root.val && q.val < root.val)
            return lowestCommonAncestor(root.left, p, q);
        // 如果都大于当前节点，那就去右子树里找
        if (p.val > root.val && q.val > root.val)
            return lowestCommonAncestor(root.right, p, q);

        // 如果都不满足，那当前节点就是公共祖先节点了
        return root;
    }