236. 二叉树的最近公共祖先

题目链接：https://leetcode.cn/problems/lowest-common-ancestor-of-a-binary-tree/

不会做，看的答案。

原理是先遍历二叉树，用hash map记录所有节点的父节点，然后分别遍历p、q节点的父节点并染色（用hash map记录节点是否染色过），遇到重复染色的节点即为二者的共同祖先节点。

/**
 * 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:
    unordered_map<int, TreeNode*> father;
    unordered_map<int, bool> colored;

    void dfs(TreeNode* root) {
        if (!root) {
            return;
        }
        if (root->left) {
            father[root->left->val] = root;
            dfs(root->left);
        }
        if (root->right) {
            father[root->right->val] = root;
            dfs(root->right);
        }
    }

    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        father[root->val] = NULL;
        dfs(root);
        while (p) {
            colored[p->val] = true;
            p = father[p->val];
        }
        while (q) {
            if (colored[q->val]) {
                return q;
            }
            q = father[q->val];
        }
        return NULL;
    }
};

/**

* 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:

unordered_map<int, TreeNode*> father;

unordered_map<int, bool> colored;

void dfs(TreeNode* root) {

if (!root) {

return;

}

if (root->left) {

father[root->left->val] = root;

dfs(root->left);

}

if (root->right) {

father[root->right->val] = root;

dfs(root->right);

}

TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {

father[root->val] = NULL;

dfs(root);

while (p) {

colored[p->val] = true;

p = father[p->val];

}

while (q) {

if (colored[q->val]) {

return q;

}

q = father[q->val];

}

return NULL;

}

};