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.

Example

Given a binary tree as follow:

      1
     / \
    2   3
       / \
      4   5

The minimum depth is 2.

Code - Java

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *   public int val;
 *   public TreeNode left, right;
 *   public TreeNode(int val) {
 *     this.val = val;
 *     this.left = this.right = null;
 *   }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */
    public int minDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        if (root.left == null && root.right == null) {
            return 1;
        }

        int left = minDepth(root.left);
        int right = minDepth(root.right);

        if (left == 0) {
            return right + 1;
        } else if (right == 0) {
            return left + 1;
        } else {
            return Math.min(left, right) + 1;
        }
    }
}