leetcode l236. Lowest Common Ancestor of a Binary Tree

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本文探讨了在给定二叉树中寻找两个节点的最低公共祖先(LCA)的问题，详细解析了算法原理，并提供了具体实现代码。通过实例演示了如何在不同情况下找到正确的LCA。

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]

在这里插入图片描述

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.

/**
 * 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(hasNode(root -> left, p) && hasNode(root -> left, q)){
            return lowestCommonAncestor(root -> left, p, q);
        }
        if(hasNode(root -> right, p) && hasNode(root -> right, q)){
            return lowestCommonAncestor(root -> right, p, q);
        }
        return root;
    }
    bool hasNode(TreeNode* root, TreeNode *p){
        if(root == NULL){
            return false;
        }
        if(root == p){
            return true;
        }
        return hasNode(root -> left, p) || hasNode(root -> right, p);
    }
};