Algorithm template of BFS

1. Basic template

    void bfs(TreeNode* root, vector<vector<int>> &result, queue<TreeNode*> &q) {
        if(root == NULL) {  //判空
            return;
        }
        
        q.push(root);
   
        while(!q.empty()) {  //队列作为条件条件进行循环
            TreeNode* node = q.front();
            q.pop();

            if(node->left != NULL){
                q.push(node->left);
            } 
            if(node->right != NULL){
                q.push(node->right);
            }
        }
    }

 

2. Examples of common writing

LeetCode  102. Level Traversal of Binary Tree

    void bfs(TreeNode* root, vector<vector<int>> &result, queue<TreeNode*> &q) {
        if(root == NULL) {
            return;
        }
        queue<int> dep;  //层  
        q.push(root);
        dep.push(0);
        while(!q.empty()) {
            TreeNode* node = q.front();
            q.pop();
            int d = dep.front(); 
            dep.pop();
            if(d == result.size()) {
                result.emplace_back();
            }
            result[d].push_back(node->val);
            if(node->left != NULL){
                q.push(node->left);
                dep.push(d + 1);
            } 
            if(node->right != NULL){
                q.push(node->right);
                dep.push(d + 1);
            }
        }
    }
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        queue<TreeNode*> q;
        bfs(root, result, q);
        return result;
    }