leetcode 102. Binary Tree Level Order Traversal 二叉樹層次遍歷-遞歸、非遞歸

https://leetcode.com/problems/binary-tree-level-order-traversal/

 

1.遞歸

/**
 * 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:
    vector<vector<int>> levelOrder(TreeNode* root) {
        bfs(root, 1);
        return result;
    }
    vector<vector<int>> result;
    void bfs(TreeNode* root, int depth){//用深度的編號來記錄層
        if(root==nullptr) return ;
        if( result.size() < depth){
            vector<int> tmp;
            result.push_back(tmp);
        }
        result[depth-1].push_back(root->val);
        
        if(root->left != nullptr)
            bfs(root->left, depth+1);//子節點深度加1
        
        if(root->right!=nullptr)
            bfs(root->right, depth+1);//子節點深度加1
    }
};

2.非遞歸

/**
 * 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:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        queue<TreeNode*> a, b; //用兩個隊列交替存儲對方的子節點
        if(root!=nullptr){
            a.push(root);
        }
        
        while(!a.empty() || !b.empty()){
            vector<int> tmp;
            while(!a.empty()){
                TreeNode* p = a.front();
                a.pop();
                tmp.push_back(p->val);
                if(p->left!=nullptr)
                    b.push(p->left);
                if(p->right!=nullptr)
                    b.push(p->right);
            }
            if(!tmp.empty())
                result.push_back(tmp);

            vector<int> second;
            while(!b.empty()){
                TreeNode* p = b.front();
                b.pop();
                second.push_back(p->val);
                if(p->left!=nullptr)
                    a.push(p->left);
                if(p->right!=nullptr)
                    a.push(p->right);
            }
            if(!second.empty())
                result.push_back(second);
        }
        return result;
    }
};