LintCode 71. Binary Tree Zigzag Level Order Traversal

原题

LintCode 71. Binary Tree Zigzag Level Order Traversal

Description

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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

代码

class Solution {
public:
    /*
    * @param root: A Tree
    * @return: A list of lists of integer include the zigzag level order traversal of its nodes' values.
    */
    vector> zigzagLevelOrder(TreeNode * root) {
        // write your code here
        helper(root, 0);
        return ans;
    }
private:
    vector> ans;
    void helper(TreeNode * root, int depth) {
        if (root == NULL) return;
        if (ans.size() < depth + 1) {
            ans.push_back(vector());
        }
        if (depth % 2) {
            ans[depth].insert(ans[depth].begin(), root->val);
        } else {
            ans[depth].push_back(root->val);
        }
        helper(root->left, depth + 1);
        helper(root->right, depth + 1);
    }
};