层次遍历

思路

按照行层次遍历，这算是基本功了。

代码

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
    vector<vector<int> > Print(TreeNode* pRoot) {
        vector<vector<int> > res;
        if(!pRoot)
            return res;
        queue<TreeNode*> Q;
        bool flag = false;  //是否需要逆序
        Q.push(pRoot);
        while(!Q.empty())
        {
            vector<int> row;
            queue<TreeNode*> QHelp;
            while(!Q.empty())
            {
                TreeNode* node = Q.front();
                Q.pop();
                row.push_back(node->val);
                if(node->left)
                    QHelp.push(node->left);
                if(node->right)
                    QHelp.push(node->right);                
            }
            Q = QHelp;
            if(flag)
            {
                //逆序
                reverse(row.begin(),row.end());
            }
            res.push_back(row);
            flag = !flag;   //这里容易toggle逻辑容易写错 是每次都需要取反的 而且取反符号是按照值取反！
        }
        return res;
    }

};