/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var minDepth = function(root) {
    // base case
    if (!root) return 0;
    let q = [];
    q.push(root);
    let depth = 1;
    // approach using BFS
    while (q.length !==0) {
        let level = q.length;
        for (let i = 0; i < level; i++) {
            let curNode = q.shift();
            // leaf
            if (!curNode.left && !curNode.right) return depth;
            if (curNode.left) {
                q.push(curNode.left);
            }
            if (curNode.right) {
                q.push(curNode.right);
            }
        }
        depth++;
    }
};