学步园

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:

Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

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

思路：这道题同样可以用层序遍历的思想，定义一个队列Q，指针front,rear分别记录每一层节点在Q中的起始坐标和终点坐标，然后遍历取得节点的值即可。

class Solution {
public:
    vector<vector<int> > levelOrder(TreeNode *root) {
        vector<vector<int> > aa;
        vector<int> line;
        int front = 0, rear = -1;
        int index,i;
        if(root == NULL)
        {
            return aa;
        }     
        vector<TreeNode*> Q;
        Q.push_back(root);
        rear = Q.size();
        line.push_back(root->val);
        aa.push_back(line);
        while(front < rear)
        {
            index = rear;
            line.clear();
            for(i=front; i<index; i++)
            {
                if (Q[i]->left != NULL)
                {
                    Q.push_back(Q[i]->left);
                    line.push_back(Q[i]->left->val);
                }   
                if (Q[i]->right != NULL)
                {
                    Q.push_back(Q[i]->right);
                    line.push_back(Q[i]->right->val);
                }  
            }  
            rear = Q.size();
            if (rear != index)
            {
                aa.push_back(line);
            }
            front = index;  
        }  
        return aa;
    }
};