111. Minimum Depth of Binary Tree

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:

Ignore null children.

/**
 * 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 !== null && root.right !== null) {
    return Math.min(minDepth(root.left), minDepth(root.right)) + 1
  } else if (root.left !== null) {
    return minDepth(root.left) + 1
  } else {
    return minDepth(root.right) + 1
  }
};

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