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]
]

层序遍历一颗二叉树，借助一个队列就可以了

为了偷懒简写，注意我定义了几个#define 字符替换

代码比较易懂，我就不解释了。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
     #define vi vector<int>
#define vvi vector<vi >
#define tr TreeNode 
class Solution {
public:


vvi levelOrder(tr* root)
{
    	vvi ans;
	if (!root)
		return ans;

	vi curLevel;
	queue<TreeNode*> Q;
	Q.push(root);
	int cur=1,next=0;
	while(!Q.empty())
	{
		while(cur--)
		{
			tr* tmp=Q.front();
			Q.pop();
			curLevel.push_back(tmp->val);
			if (tmp->left)
			{	
				Q.push(tmp->left);
				next++;
			}
			if (tmp->right)
			{
				Q.push(tmp->right);
				next++;
			}
		}
		ans.push_back(curLevel);
		curLevel.clear();
		cur=next;
		next=0;
	}
	return ans;
}	

};