Problem

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its minimum depth = 2.

Solution 1

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var minDepth = function(root) {
  if (!root) return 0;
  if (!root.left) return minDepth(root.right) + 1;
  if (!root.right) return minDepth(root.left) + 1;
  return Math.min(minDepth(root.left), minDepth(root.right)) + 1;
};

Explain:

dfs

Complexity:

• Time complexity : O(n).
• Space complexity : O(1).

Solution 2

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var minDepth = function(root) {
  if (!root) return 0;

  var queue = [[root, 1]];
  var item = null;
  var node = null;
  var level = 0;

  while (queue.length) {
    item = queue.shift();
    node = item[0];
    level = item[1];
    if (!node.left && !node.right) return level;
    if (node.left) queue.push([node.left, level + 1]);
    if (node.right) queue.push([node.right, level + 1]);
  }
};

Explain:

bfs

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).