[Data structure] BFS code template

Source: Qin Chao

Python

def BFS(graph, start, end):
    visited = set()
	queue = [] 
	queue.append([start]) 
	while queue: 
		node = queue.pop() 
		visited.add(node)
		process(node) 
		nodes = generate_related_nodes(node) 
		queue.push(nodes)
	# other processing work 
	...

Java

public class TreeNode {
    
    
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
    
    
        val = x;
    }
}

public List<List<Integer>> levelOrder(TreeNode root) {
    
    
    List<List<Integer>> allResults = new ArrayList<>();
    if (root == null) {
    
    
        return allResults;
    }
    Queue<TreeNode> nodes = new LinkedList<>();
    nodes.add(root);
    while (!nodes.isEmpty()) {
    
    
        int size = nodes.size();
        List<Integer> results = new ArrayList<>();
        for (int i = 0; i < size; i++) {
    
    
            TreeNode node = nodes.poll();
            results.add(node.val);
            if (node.left != null) {
    
    
                nodes.add(node.left);
            }
            if (node.right != null) {
    
    
                nodes.add(node.right);
            }
        }
        allResults.add(results);
    }
    return allResults;
}

C/C++

void bfs(Node* root) {
    
    
  map<int, int> visited;
  if(!root) return ;

  queue<Node*> queueNode;
  queueNode.push(root);

  while (!queueNode.empty()) {
    
    
    Node* node = queueNode.top();
    queueNode.pop();
    if (visited.count(node->val)) continue;
    visited[node->val] = 1;

    for (int i = 0; i < node->children.size(); ++i) {
    
    
        queueNode.push(node->children[i]);
    }
  }

  return ;
}

JavaScript

const bfs = (root) => {
    
    
  let result = [], queue = [root]
  while (queue.length > 0) {
    
    
    let level = [], n = queue.length
    for (let i = 0; i < n; i++) {
    
    
      let node = queue.pop()
      level.push(node.val) 
      if (node.left) queue.unshift(node.left)
      if (node.right) queue.unshift(node.right)
    }
    result.push(level)
  }
  return result
};