https://leetcode.com/problems/minimum-depth-of-binary-tree/description/

MinimumDepthOfBinaryTree.java

class MinimumDepthOfBinaryTree {

    int minDepth = 0;
    int currentDepth = 0;
    public static void main(String[] args) {
        TreeNode node9 = new TreeNode(9);
        TreeNode node15 = new TreeNode(15);
        TreeNode node7 = new TreeNode(7);
        TreeNode node20 = new TreeNode(20,node15,node7);
        TreeNode node3 = new TreeNode(3,node9,node20);
        System.out.println(new MinimumDepthOfBinaryTree().minDepth(node7));
    }

    public int minDepth(TreeNode root) {
        if(root!=null){
            currentDepth++;
            if(root.left==null && root.right==null){
                if(minDepth==0)
                    minDepth = currentDepth;
                else
                    minDepth = Math.min(currentDepth, minDepth);
            }else{
                if(root.left!=null)
                    minDepth(root.left);
                if(root.right!=null)
                    minDepth(root.right);
            }
            currentDepth--;
        }
        return minDepth;
    }
}

class TreeNode {
int val;
  TreeNode left;
  TreeNode right;
  TreeNode() {}
  TreeNode(int val) { this.val = val; }
  TreeNode(int val, TreeNode left, TreeNode right) {
      this.val = val;
      this.left = left;
      this.right = right;
  }
}