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/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func minDepth(root *TreeNode) int {
    if root == nil {
        return 0
    }
    
    return dfs(root)
}

func dfs(root *TreeNode) int {
    if root.Left == nil && root.Right == nil {
        return 1
    }
    
    if root.Left == nil {
        return dfs(root.Right) + 1
    }
    if root.Right == nil {
        return dfs(root.Left) + 1
    }

    return min(dfs(root.Left), dfs(root.Right)) + 1
}