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题目

• 原题连接：236. 二叉树的最近公共祖先

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1- 思路

模式识别

• 模式1：二叉树最近公共祖先 ——> 递归 + 判断

递归思路，分情况判断：

• 1.参数及返回值
  • public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q)
• 2.递归终止条件
  • 遇到 null、 root == p 、root = q 的情况，此时递归结束。
• 3.单层递归逻辑 4 种情况
  • 递归 left 赋值
  • 递归 right 赋值
  • ① 左右都为 null、此时返回 null
  • ② 左不 **null**、右 **null** 返回左
  • ③ 左**null**、右不 **null** 返回右
  • ④ 直接返回 root

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2- 实现

⭐236. 二叉树的最近公共祖先——题解思路

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root==null || root==p || root==q){
            return root;
        }

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

        if(left==null && right==null){
            return null;
        }else if(left!=null && right==null){
            return left;
        }else if(left==null && right!=null){
            return right;
        }else{
            return root;
        }
    }
}

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3- ACM实现

public class lowestCommonAncestor {


    static class TreeNode{
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(){}
        TreeNode(int x){
            val = x;
        }
    }


    public static TreeNode build(Integer[] nums){
        TreeNode root = new TreeNode(nums[0]);
        Queue<TreeNode> queue =new LinkedList<>();
        queue.offer(root);
        int index = 1;
        while(!queue.isEmpty() && index < nums.length){
            TreeNode node = queue.poll();

            if(nums[index]!=null && index<nums.length){
                node.left = new TreeNode(nums[index]);
                queue.offer(node.left);
            }
            index++;
            if(nums[index]!=null && index<nums.length){
                node.right = new TreeNode(nums[index]);
                queue.offer(node.right);
            }
            index++;
        }
        return root;
    }


    public static TreeNode findOne(TreeNode root,int val){
        if(root==null) return root;
        if(root.val == val) return root;

        TreeNode left = findOne(root.left,val);
        if(left!=null) return left;
        return findOne(root.right,val);
    }


    public static TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root==null || root==p || root==q){
            return root;
        }

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

        if(left==null && right==null){
            return null;
        }else if(left!=null && right==null){
            return left;
        }else if(left==null && right!=null){
            return right;
        }else{
            return root;
        }
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("输入二叉树构造数组");
        String input = sc.nextLine();
        input = input.replace("[","");
        input = input.replace("]","");
        String[] parts = input.split(",");
        Integer[] nums = new Integer[parts.length];
        for(int i = 0 ; i <parts.length;i++){
            if(!parts[i].equals("null")){
                nums[i] = Integer.parseInt(parts[i]);
            }else{
                nums[i] = null;
            }
        }

        TreeNode root = build(nums);
        System.out.println("输入val1");
        TreeNode p = findOne(root,sc.nextInt());

        System.out.println("输入val2");
        TreeNode q = findOne(root,sc.nextInt());

        TreeNode res = lowestCommonAncestor(root,p,q);
        System.out.println("公共父节点值为"+res.val);
    }
}

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