Leetcode-runtime error: reference binding to null pointer of type 'struct value_type'

leetcode-102. hierarchical binary tree traversal

Modify the error code before

class Solution {
public:vector<vector<int>> levelOrder(TreeNode* root) {
	vector<vector<int>> res;                
	queue<TreeNode*> LL;        
	queue<TreeNode*> RR;
	
	int lev=0;                
	if(root) LL.push(root);
	
	while(!LL.empty()||!RR.empty()){            
        	if(!LL.empty()){
			while(!LL.empty()){                    
			        TreeNode* tmp=LL.front();                    
			        LL.pop();                    
			        res[lev].push_back(tmp->val);                    
			        if(tmp->left)   RR.push(tmp->left);                    
			        if(tmp->right)  RR.push(tmp->right);                
		        }                
		        lev++;            
		}            
	        else if(!RR.empty()){                
		   	while(!RR.empty()){                    
			        TreeNode* tmp=RR.front();                    
			        RR.pop();                    
			        res[lev].push_back(tmp->val);                    
			        if(tmp->left)   LL.push(tmp->left);                    
			        if(tmp->right)  LL.push(tmp->right);                
		        }                
		        lev++;            
		}
	}        
        return res;
}
};

Thought problem solving: traversing each with a single queue, the lower left and right subtrees to another queue.

Cause: irregular array using the vector, RES is the vector <vector <>> type, where the outer layer is not inserted in the vector, the inner layer can not be used. See below:

if(){
	while(){
		```
		res[lev].push_back(tmp->val);
		```
	}
	lev++;
}

Amended as follows: first through the results stored in a temporary vector variables, the last save. See below:

if(){
	vector<int> r;
	while(){
		```
		r.push_back(tmp->val);
		```
	}
	res.push_back(r);
}

AC modified the code:

/** * Definition for a binary tree node. 
* struct TreeNode { 
* *     int val; 
* *     TreeNode *left; 
* *     TreeNode *right; 
* *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 
* * }; 
* */
class Solution {
public:    vector<vector<int>> levelOrder(TreeNode* root) {
	vector<vector<int>> res;                
	queue<TreeNode*> LL;        
	queue<TreeNode*> RR;                
	if(root) LL.push(root);
	while(!LL.empty()||!RR.empty()){            
        	if(!LL.empty()){                
        		vector<int> r;                
		        while(!LL.empty()){                    
			        TreeNode* tmp=LL.front();                    
			        LL.pop();                    
			        r.push_back(tmp->val);                    
			        if(tmp->left)   RR.push(tmp->left);                    
			        if(tmp->right)  RR.push(tmp->right);                
		        }                
		        res.push_back(r);            
		}            
	        else if(!RR.empty()){                
		        vector<int> r;                
		        while(!RR.empty()){                    
			        TreeNode* tmp=RR.front();                    
			        RR.pop();                    
			        r.push_back(tmp->val);                    
			        if(tmp->left)   LL.push(tmp->left);                    
			        if(tmp->right)  LL.push(tmp->right);                
		        }                
		        res.push_back(r);            
		}
	}        
        return res;
}
};

Browse other blog, mentioned more problems (recommended check):

1. For empty Input [] can not be solved.
2.vector array initialization error, the code out of range.