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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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