树的DFS

Depth-first search (DFS) is an algorithm for traversing or searching tree or graph data structures. One starts at the root (selecting some arbitrary node as the root in the case of a graph) and explores as far as possible along each branch before backtracking.

代码如下：

/**
 * @typedef {Object} Callbacks
 * @property {function(node: BinaryTreeNode, child: BinaryTreeNode): boolean} allowTraversal -
 *   Determines whether DFS should traverse from the node to its child.
 * @property {function(node: BinaryTreeNode)} enterNode - Called when DFS enters the node.
 * @property {function(node: BinaryTreeNode)} leaveNode - Called when DFS leaves the node.
 */

/**
 * @param {Callbacks} [callbacks]
 * @returns {Callbacks}
 */
function initCallbacks(callbacks = {}) {
  const initiatedCallback = callbacks;

  const stubCallback = () => {};
  const defaultAllowTraversal = () => true;

  initiatedCallback.allowTraversal = callbacks.allowTraversal || defaultAllowTraversal;
  initiatedCallback.enterNode = callbacks.enterNode || stubCallback;
  initiatedCallback.leaveNode = callbacks.leaveNode || stubCallback;

  return initiatedCallback;
}

/**
 * @param {BinaryTreeNode} node
 * @param {Callbacks} callbacks
 */
export function depthFirstSearchRecursive(node, callbacks) {
  callbacks.enterNode(node);

  // Traverse left branch.
  if (node.left && callbacks.allowTraversal(node, node.left)) {
    depthFirstSearchRecursive(node.left, callbacks);
  }

  // Traverse right branch.
  if (node.right && callbacks.allowTraversal(node, node.right)) {
    depthFirstSearchRecursive(node.right, callbacks);
  }

  callbacks.leaveNode(node);
}

/**
 * @param {BinaryTreeNode} rootNode
 * @param {Callbacks} [callbacks]
 */
export default function depthFirstSearch(rootNode, callbacks) {
  depthFirstSearchRecursive(rootNode, initCallbacks(callbacks));
}