1、Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

class Solution {
public:
    int maxDepth(TreeNode *root) {
          inMax = 0;
          int depth = 0;
          if(root){
              maxDepth(root,depth);
          }
          return inMax;
    }
    void maxDepth(TreeNode* root, int depth){
        ++depth;
        if(!root->left && !root->right){
            if(depth >inMax){
                inMax = depth;
            }
        }
        if(root->left){
            maxDepth(root->left,depth);
        }
        if(root->right){
            maxDepth(root->right,depth);
        }
    }
    int inMax;
};

2、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).

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]
]

分析：有栈来保存结点，第一次先放左孩子后放右孩子，第二次先放右孩子再放左孩子。

class Solution {
public:
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        vector<vector<int> > result;
        stack<TreeNode*> stackNode;
        stack<TreeNode*> newStackNode;
        vector<int> nodeVec;
        if(root){
            stackNode.push(root);
            while(!stackNode.empty()){
                nodeVec.clear();
                 while(!stackNode.empty()){
                    TreeNode* node = stackNode.top();
                    stackNode.pop();
                    nodeVec.push_back(node->val);
                    if(node->left){
                        newStackNode.push(node->left);
                    }
                    if(node->right){
                        newStackNode.push(node->right);
                    }
                }
                result.push_back(nodeVec);
                nodeVec.clear();
                while(!newStackNode.empty()){
                    TreeNode* node = newStackNode.top();
                    newStackNode.pop();
                    nodeVec.push_back(node->val);
                    if(node->right){
                        stackNode.push(node->right);
                    }
                    if(node->left){
                        stackNode.push(node->left);
                    }
                }
                if(!nodeVec.empty()){
                    result.push_back(nodeVec);
                }
            }
        }
        return result;
    }
};