面试题68

题目描述：

测试用例：

思路：

• 显而易见这题使用递归算法找左右子树

基本流程：

1. 递归结束条件：

   1. 传入参数root为null，返回root

   2. 传入参数root为p或q节点，返回root

2. 递归左右子树，获得递归结果left，right

3. 递归结果的四种情况：

   1. 左右子树都不为null，返回根root就是最近公共祖先

   2. 左子树为null，返回右子树，由于左子树都不包含p或q一定不为公共祖先，右子树可能为公共祖先或p或q或null

   3. 同理：右子树为null，返回左子树。

   4. 补充：情况2和情况3包含了三种情况：

      1. 左子树为null，右子树不为null

      2. 右子树为null，左子树不为null

      3. 左右子树都为null

代码：

import java.util.Objects;

public class P236LowestCommonAncestorOfABinaryTree {

    static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }

    //root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
    public static void main(String[] args) {
        Solution solution = new P236LowestCommonAncestorOfABinaryTree().new Solution();
        // TO TEST
        TreeNode root = new TreeNode(3);
        TreeNode root1 = new TreeNode(5);
        TreeNode root2 = new TreeNode(1);
        TreeNode root3 = new TreeNode(6);
        TreeNode root4 = new TreeNode(2);
        TreeNode root5 = new TreeNode(0);
        TreeNode root6 = new TreeNode(8);
        TreeNode root7 = new TreeNode(7);
        TreeNode root8 = new TreeNode(4);


        root.left = root1;
        root.right = root2;
        root1.left = root3;
        root1.right = root4;
        root2.left = root5;
        root2.right = root6;
        root4.left = root7;
        root4.right = root8;

        root3.left = root3.right = root5.left = root5.right = root6.left = root6.right = root7.left = root7.right = root8.left = root8.right = null;

        System.out.println(solution.lowestCommonAncestor(root, root1, root2).val);
    }
    //leetcode submit region begin(Prohibit modification and deletion)

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     * int val;
     * TreeNode left;
     * TreeNode right;
     * TreeNode(int x) { val = x; }
     * }
     */
    class Solution {
        public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {

            if (Objects.isNull(root) || root == p || root == q)
                return root;

            TreeNode left = lowestCommonAncestor(root.left, p, q);
            TreeNode right = lowestCommonAncestor(root.right, p, q);

            if (Objects.nonNull(left) && Objects.nonNull(right)) return root;
            else if (Objects.isNull(left)) return right;
            else return left;
        }
    }
}