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

落日映苍穹つ 2022-07-12 11:49 89阅读 0赞

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;
            }
    
    
        }
    }