[Swift]LeetCode236. 二叉树的最近公共祖先 | Lowest Common Ancestor of a Binary Tree

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Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Given the following binary tree:  root = [3,5,1,6,2,0,8,null,null,7,4]

 

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

 

Note:

• All of the nodes' values will be unique.
• p and q are different and both values will exist in the binary tree.

---

给定一个二叉树, 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）。”

例如，给定如下二叉树:  root = [3,5,1,6,2,0,8,null,null,7,4]

 

示例 1:

输入: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
输出: 3
解释: 节点 5 和节点 1 的最近公共祖先是节点 3。

示例 2:

输入: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
输出: 5
解释: 节点 5 和节点 4 的最近公共祖先是节点 5。因为根据定义最近公共祖先节点可以为节点本身。

 

说明:

• 所有节点的值都是唯一的。
• p、q 为不同节点且均存在于给定的二叉树中。

---

递归

class Solution {
    func lowestCommonAncestor(_ root: TreeNode?, _ p: TreeNode?,_ q: TreeNode?) -> TreeNode? {
        if root!.val == nil || root!.equal(p) || root!.equal(q)
        {
            return root
        }
        
        var left:TreeNode? = lowestCommonAncestor(root!.left, p, q)
        if left!.val != nil && left!.equal(p) && left!.equal(q)
        {
            return left
        }
        
        var right:TreeNode? = lowestCommonAncestor(root!.right, p , q)
        if left!.val != nil && right!.val != nil
        {
            return root
        }
        return left!.val != nil ? left : right
    }
}
public class TreeNode {
    public var val: Int
     public var left: TreeNode?
     public var right: TreeNode?
     public init(_ val: Int) {
        self.val = val
        self.left = nil
        self.right = nil
    }
    
    func equal(_ root: TreeNode?)-> Bool 
    {
        return (self.val == root!.val) && (self.left!.val == root!.left!.val) && (self.right!.val == root!.right!.val)
    }    
 }

---

C++：4ms

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
static const auto _____ = []()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    return nullptr;
}();
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (root == NULL || root == p || root == q) return root;
        TreeNode* ptr1 = lowestCommonAncestor(root->left, p, q);
        TreeNode* ptr2 = lowestCommonAncestor(root->right, p, q);
        if (ptr1 && ptr2) return root;
        return ptr1 ? ptr1 : ptr2;
    }
};

---

C++：12ms

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if(!root||p==root||q==root) return root;
        TreeNode *left = lowestCommonAncestor(root->left, p, q);
        TreeNode *right = lowestCommonAncestor(root->right, p, q);
        if (left && right) return root;
        return left ? left : right;
        
    }
};