leetcode:235. Lowest Common Ancestor of a Binary Search Tree

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

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

    _______6______   /              \ ___2__          ___8__/      \        /      \0      _4       7       9      /  \     3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {        TreeNode[] parent = new TreeNode[2];        dfs(root, root, p, q, parent);        return parent[0] == parent[1] ? parent[0] : null;    }    static void dfs(TreeNode root, TreeNode cur, TreeNode p, TreeNode q, TreeNode[] parent) {        if (cur == null) {            return;        }        if (cur == p) {            parent[0] = cur;        }        if (cur == q) {            parent[1] = cur;        }        dfs(cur, cur.left, p, q, parent);        dfs(cur, cur.right, p, q, parent);        if (parent[0] != null && parent[1] != null && parent[0] == parent[1]) {            return;        }        if (parent[0] == cur) {            parent[0] = root;        }        if (parent[1] == cur) {            parent[1] = root;        }    }}