[Leetcode]Binary Tree Zigzag Level Order Traversal

最新推荐文章于 2021-02-05 11:10:39 发布

原创最新推荐文章于 2021-02-05 11:10:39 发布 · 343 阅读

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#leetcode #算法

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本文介绍了一种算法，用于实现二叉树节点值的锯齿形层序遍历。提供了两种实现方式：一种是使用队列和栈进行广度优先搜索；另一种是递归方式实现。通过这两种方法，可以有效地获取二叉树各层节点值，并按要求顺序返回。

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Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

/**
 * 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:
    /*algorithm: BFS
    use stack to reverse instead of reverse
    */
    void bfs(vector<vector<int>>&result,TreeNode* root){
        queue<TreeNode*>Q;
        if(root)Q.push(root);
        bool flip = false;
        while(!Q.empty()){
            int size = Q.size();
            vector<int>path;
            stack<int>stk;
            for(int i = 0;i < size;i++){
                TreeNode* t = Q.front();Q.pop();
                if(t->left)Q.push(t->left);
                if(t->right)Q.push(t->right);
                if(flip)stk.push(t->val);
                else path.push_back(t->val);
            }
            if(flip){
                while(!stk.empty()){
                    path.push_back(stk.top());
                    stk.pop();
                }
            }
            flip = !flip;
            result.push_back(path);
        }
    }
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>>result;
        vector<int>path;
        bfs(result,root);
        return result;
    }
};

/**
 * 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:
    /*algorithm:bfs recursive 
    */
    void preorder(vector<vector<int>>&result,TreeNode* root,int level){
         if(!root)return;
         if(result.size() <= level)
            result.push_back(vector<int>());
        if(level%2)//odd
            result[level].insert(result[level].begin(),root->val);
        else
            result[level].push_back(root->val);
        preorder(result,root->left,level+1);
        preorder(result,root->right,level+1);
    }
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
            vector<vector<int>>result;
            preorder(result,root,0);
            return result;
    }
};