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[LeetCode] Binary Tree Zigzag Level Order Traversal 二叉树的之字形层序遍历

时间:2015-02-21 15:26:40      阅读:185      评论:0      收藏:0      [点我收藏+]

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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},

    3
   /   9  20
    /     15   7

 

return its zigzag level order traversal as:

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

 

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

 

这道二叉树的之字形层序遍历是之前那道[LeetCode] Binary Tree Level Order Traversal 二叉树层序遍历的变形,不同之处在于一行是从左到右遍历,下一行是从右往左遍历,交叉往返的之字形的层序遍历。根据其特点我们用到栈的后进先出的特点,这道题我们维护两个栈,相邻两行分别存到两个栈中,进栈的顺序也不相同,一个栈是先进左子结点然后右子节点,另一个栈是先进右子节点然后左子结点,这样出栈的顺序就是我们想要的之字形了,代码如下:

 

/**
 * 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> >res;
        if (!root) return res;
        stack<TreeNode*> s1;
        stack<TreeNode*> s2;
        s1.push(root);
        vector<int> out;
        while (!s1.empty() || !s2.empty()) {
            while (!s1.empty()) {
                TreeNode *cur = s1.top();
                s1.pop();
                out.push_back(cur->val);
                if (cur->left) s2.push(cur->left);
                if (cur->right) s2.push(cur->right);
            } 
            if (!out.empty()) res.push_back(out);
            out.clear();
            while (!s2.empty()) {
                TreeNode *cur = s2.top();
                s2.pop();
                out.push_back(cur->val);
                if (cur->right) s1.push(cur->right);
                if (cur->left) s1.push(cur->left);
            }
            if (!out.empty()) res.push_back(out);
            out.clear();
        }
        return res;
    }
};

 

比如对于题干中的那个例子:

    3
   /   9  20
    /     15   7

我们来看每一层两个栈s1, s2的情况:

s1:  3

s2:

 

s1:  

s2:  9  20

 

s1:  7  15

s2: 

 

[LeetCode] Binary Tree Zigzag Level Order Traversal 二叉树的之字形层序遍历

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原文地址:http://www.cnblogs.com/grandyang/p/4297009.html

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