Leecode no.235 二叉搜索树的最近公共祖先

package tree;

/**
 * 235. 二叉搜索树的最近公共祖先
 * 给定一个二叉搜索树, 找到该树中两个指定节点的最近公共祖先。
 *
 * 百度百科中最近公共祖先的定义为：“对于有根树 T 的两个结点 p、q，最近公共祖先表示为一个结点 x，满足 x 是 p、q 的祖先且 x 的深度尽可能大（一个节点也可以是它自己的祖先）。”
 *
 * @author :Tang
 * @date  2021/7/19
 */
public class LowestCommonAncestor {

    int small = 0;
    int big = 0;

    /**
     * 因为是二叉搜索树
     * 通过判断当前节点的大小来判断是不是公共祖先
     * 当前节点大于p && 小于q 说明当前节点是公共祖先
     * 如果当前节点小于p 则递归当前节点的右子节点
     * 如果当前节点大于q 则递归当前节点的左子节点
     * (这里默认认为节点p的值大于节点q的值)
     *
     * @param root
     * @param p
     * @param q
     * @return
     */
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        big = Math.max(p.val, q.val);
        small = Math.min(p.val, q.val);
        return preSearch(root);
    }

    private TreeNode preSearch(TreeNode node){
        if(node == null){
            return null;
        }
        if(node.val < small){
            return preSearch(node.right);
        }

        if(node.val > big){
            return preSearch(node.left);
        }
        return node;
    }

    public static void main(String[] args) {

        TreeNode node1 = new TreeNode(6);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(8);
        TreeNode node4 = new TreeNode(0);
        TreeNode node5 = new TreeNode(4);
        TreeNode node6 = new TreeNode(7);
        TreeNode node7 = new TreeNode(9);
        TreeNode node8 = new TreeNode(3);
        TreeNode node9 = new TreeNode(5);

        node1.left = node2;
        node1.right = node3;
        node2.left = node4;
        node2.right = node5;
        node3.left = node6;
        node3.right = node7;
        node5.left = node8;
        node5.right = node9;
        System.out.println(new LowestCommonAncestor().lowestCommonAncestor(node1, node2, node5).val);

    }
}