leetcode-Binary Tree Zigzag Level Order Traversal

最新推荐文章于 2020-07-25 07:42:38 发布

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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。从左到右再从右到左交替进行，并提供了一个C++示例实现。

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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 binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        vector<vector<int> > v;
        if(root == NULL) return v;
        queue<TreeNode*> q;
        queue<int> qlevel;
        q.push(root);
        qlevel.push(1);
        vector<int> a;
        v.push_back(a);
        int l = 1;
        vector<int> *p = &v[l-1];
        while(!q.empty())
        {
            TreeNode *node = q.front();
            int level = qlevel.front();
            if(l < level)
            {
                vector<int> b;
                v.push_back(b);
                l++;
                p = &v[l-1];
            }
            p->push_back(node->val);
            if(node->left)
            {
                q.push(node->left);
                qlevel.push(level+1);
            }
            if(node->right)
            {
                q.push(node->right);
                qlevel.push(level+1);
            }
            
            q.pop();
            qlevel.pop();
        }
        int n = v.size();
        for(int i = 1; i < n; i += 2)
        {
            reverse(v[i].begin(),v[i].end());
        }
        return v;
    }
};