目錄

• 題目鏈接
• 註意點
• 解法
• 小結

題目鏈接

Binary Tree Zigzag Level Order Traversal - LeetCode

註意點

• 不要訪問空結點

解法

解法一：遞歸，遞歸，在Binary Tree Level Order Traversal - LeetCode的基礎上將奇數層（下標從0開始）結果逆序即可。

/**
 * 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:
    typedef vector<vector<int>> v;
    typedef TreeNode* node;
    void bfs(int level,node n,v& ret)
    {
        if(ret.size() == level) ret.push_back({});
        ret[level].push_back(n->val);
        if(n->left) bfs(level+1,n->left,ret);
        if(n->right) bfs(level+1,n->right,ret);
    }
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        v ret;
        if(!root) return ret;
        bfs(0,root,ret);
        int i = 0,n = ret.size();
        for(i = 0;i < n;i++)
        {
            if(i%2 == 1) reverse(ret[i].begin(),ret[i].end());
        }
        return ret;
    }
};
 

解法二：非遞歸，同樣在Binary Tree Level Order Traversal - LeetCode的基礎上訪問到奇數層時結果逆序即可。

/**
 * 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:
    typedef vector<vector<int>> v;
    typedef TreeNode* node;
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        v ret;
        if(!root) return ret;
        int level = 0;
        queue<node> q;
        q.push(root);
        while(!q.empty())
        {
            vector<int> aLevel;
            for(int i = q.size();i > 0;--i)
            {
                node n = q.front();
                q.pop();
                aLevel.push_back(n->val);
                if(n->left) q.push(n->left);
                if(n->right) q.push(n->right);
            }
            if(level%2 == 1) reverse(aLevel.begin(),aLevel.end());
            ret.push_back(aLevel);
            level++;
        }
        return ret;
    }
};
 

小結

Binary Tree Zigzag Level Order Traversal - LeetCode