二叉树判断表兄弟表兄妹算法(递归, 深度优先)

/**
 * 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) {                     
        dfs(root, NULL, 0);
        return (depth[x] == depth[y]) && (parent[x] != parent[y]);
    }
    
    void dfs(TreeNode* root, TreeNode* p, int d) {
        if (root == NULL) return;
        depth[root->val] = d; // 记录深度
        parent[root->val] = p; // 记录父母
        dfs(root->left, root, d + 1);
        dfs(root->right, root, d + 1);
    }
 
private:
    unordered_map<int, int> depth;
    unordered_map<int, TreeNode*> parent;
};

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool isCousins(TreeNode* root, int x, int y) {
        pair<int, TreeNode*> a = dfs(root, x, 0, nullptr);
        pair<int, TreeNode*> b = dfs(root, y, 0, nullptr);
        return (a.first == b.first) && (a.second != b.second);
    }
    
private:
    pair<int, TreeNode*> dfs(TreeNode* root, int val, int depth, TreeNode* parent) {
        // 如果找不到该节点, 就返回特殊值
        if (root == nullptr) {
            return {0, nullptr}; 
        }
        if (root->val == val) {
            return {depth, parent};
        }
        auto a = dfs(root->left, val, depth + 1, root);
        if (a.first != 0) return a;
        auto b = dfs(root->right, val, depth + 1, root);
        return b;        
    }
};

/**
 * 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) {
        TreeNode* leftParent = findParent(root, nullptr, x);
        TreeNode* rightParent = findParent(root, nullptr, y);
        if (leftParent == rightParent) return false;
        int dx = depth(root, 0, x);
        int dy = depth(root, 0, y);
        return dx == dy;
    }
    
private:
    // 得到节点 v 的深度
    int depth(TreeNode* root, int curDepth, int v) {
        if (root == nullptr) return 0;
        if (root->val == v) {
            return curDepth;
        }
        int x = depth(root->left, curDepth + 1, v);
        if (x > 0) return x;
        return depth(root->right, curDepth + 1, v);
    }
    
    // 得到节点 v 的父母
    TreeNode* findParent(TreeNode* root, TreeNode* parent, int v) {
        if (root == nullptr) return nullptr;
        if (root->val == v) return parent;
        TreeNode* left = findParent(root->left, root, v);
        if (left != nullptr) return left;
        return findParent(root->right, root, v);
    }
};