[LeetCode] 103. Binary Tree Zigzag Level Order Traversal

Binary Tree Zigzag Level Order Traversal

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).
Example:

Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:
[
[3],
[20,9],
[15,7]
]

解析

同样是层次遍历，但是需要Z字形输出节点值，所以考虑使用两个栈，进栈顺序分别为先进左子结点然后右子节点和先进右子节点然后左子结点。出栈就能得到相应的Z字形序列。

代码

class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int> > res;
        if(!root) return res;
        stack<TreeNode* > v1;
        stack<TreeNode* > v2;
        v1.push(root);
        while(!v1.empty() || !v2.empty()){
            vector<int> v;
            while(!v1.empty()){
                TreeNode* node = v1.top();
                v1.pop();
                v.push_back(node->val);
                if(node->left) v2.push(node->left);
                if(node->right) v2.push(node->right);
            }
            if(!v.empty()) res.push_back(v);
            v.clear();
            while(!v2.empty()){
                TreeNode* node = v2.top();
                v2.pop();
                v.push_back(node->val);
                if(node->right) v1.push(node->right);
                if(node->left) v1.push(node->left);  
            }
            if(!v.empty()) res.push_back(v);
            v.clear();
        }
        return res;
    }
};

参考

http://www.cnblogs.com/grandyang/p/4297009.html