993. Cousins in Binary Tree

bfs O(n) time O(n) space

/**
 * 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:
    bool isCousins(TreeNode* root, int x, int y) {
        queue<TreeNode *> q;
        q.push(root);
        while (!q.empty()) {
            unordered_set<int> s;
            for (int i = q.size(); i > 0; --i) {
                auto t = q.front(); q.pop();
                if (t->left && t->right) {
                    unordered_set<int> temp{t->left->val, t->right->val};
                    if (temp.count(x) && temp.count(y)) return false; // 如果x和y共父，则非法
                }
                s.insert(t->val);
                if (t->left) {
                    q.push(t->left);
                }
                if (t->right) {
                    q.push(t->right);
                }
            }
            if (s.count(x) && s.count(y)) return true; // 搜集当前层所有点，看x和y是不是都存在
            if (s.count(x) || s.count(y)) return false;
        }
        return false;
    }
};