C C++ LeetCode题解在二叉树中增加一行示例详解

/**
 * 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:
    TreeNode* addOneRow(TreeNode* root, int val, int depth) {
        // 特判 depth = 1 的情况
        if(depth == 1){
            TreeNode *res = new TreeNode(val);
            res->left = root;
            return res;
        }
        // k 记录当前层数
        int k = 1;
        queue<TreeNode*> que;
        while(que.size()) que.pop();
        que.push(root);
        // bfs层序遍历
        while(que.size()){
            int n = que.size();
            // 遍历到 depth - 1 层时开始插入元素 val
            if(k == depth - 1){
                for(int i = 0; i < n; i++){
                    TreeNode *now = que.front();
                    que.pop();
                    TreeNode *l = new TreeNode(val, now->left, NULL);
                    TreeNode *r = new TreeNode(val, NULL, now->right);
                    now->left = l;
                    now->right = r;
                }
                // 插入完成后跳出
                break;
            }
            // 压入下一层节点元素
            for(int i = 0; i < n; i++){
                TreeNode *now = que.front();
                que.pop();
                if(now->left != NULL) que.push(now->left);
                if(now->right != NULL) que.push(now->right);
            }
            k++;
        }
        return root;
    }
};

/**
 * 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:
    TreeNode* addOneRow(TreeNode* root, int val, int depth) {
        if(root == nullptr) return root;
        // 特判 depth = 1 的情况
        if(depth == 1){
            return new TreeNode(val, root, nullptr);
        }
        // 当 depth 到第 2 层时表示 在当前层的下一层插入节点 val
        if(depth == 2){
            root->left = new TreeNode(val, root->left, nullptr);
            root->right = new TreeNode(val, nullptr, root->right);
            return root;
        }
        // 否则一直递归
        else{
            root->left = addOneRow(root->left, val, depth - 1);
            root->right = addOneRow(root->right, val, depth - 1);
        }
        return root;
    }
};