思路

广度优先遍历，在遍历过程中遇到叶子节点停止遍历返回层级

代码

/**
 * 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) {
    const bfs = (n) => {
      if(n){
      //新建一个队列，根节点入队
          const q = [[root,1]]
          while(q.length){
          //根节点出对后，左右节点挨个入队
              const [a,l] = q.shift()
              if(!a.left && !a.right){
                  return l
              }
              if(a.left) q.push([a.left,l+1])
              if(a.right) q.push([a.right,l+1])
          }
      }
      return 0;
    }
    return bfs(root)
};

复杂度

时间:O(n)

空间:O(n)