LeetCode algorithm binary tree—236. Recent common ancestor of binary tree

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236. Recent common ancestor of binary tree

Code:

operation result: 

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Given a binary tree, find the nearest common ancestor of two specified nodes in the tree.

The definition of the nearest common ancestor in Baidu Encyclopedia is: "For two nodes p and q of a rooted tree T, the nearest common ancestor is represented as a node x, such that x is the ancestor of p and q and the depth of x is as large as possible ( a A node can also be its own ancestor )."

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
 Output: 3
 Explanation: Node 5 and node 1 ’s nearest common ancestor is node3 。

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
 Output: 5
 Explanation: Node 5 and node 4 ’s nearest common ancestor is node 5 。because By definition the nearest common ancestor node can be the node itself.

Example 3:

Input: root = [1,2], p = 1, q = 2
 Output: 1

hint:

• The number of nodes in the tree is  [2, 105] within the range.
• -109 <= Node.val <= 109
• All  Node.val 互不相同 .
• p != q
• p and  q both exist in the given binary tree.

Code:

/**
 * 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) {
        // p=root ，则 q 在 root 的左或右子树中；
        // q=root ，则 p 在 root 的左或右子树中；
        // 即题目提示：一个节点也可以是它自己的祖先
        if(root==null||root==p||root==q) return root;
        // 不是则让左右节点继续往下递归，在本层递归看来这步是给left赋值，看看有没有p,q在左子树上
        TreeNode left=lowestCommonAncestor(root.left,p,q);
        // 与上一步一样
        TreeNode right=lowestCommonAncestor(root.right,p,q);
        // 如果left 和 right都不为空，说明此时root就是最近公共节点
        // 如果left为空，right不为空，就返回right，说明目标节点是通过right返回的，反之亦然
        if(left != null && right != null) return root;
        if(left==null) return right;
        return left;
    }
}

operation result: