111. Minimum Depth of Binary Tree

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.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

return its minimum depth = 2.

time: O(n), space: O(height)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    
    public int minDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
        if(root.left == null) {
            return minDepth(root.right) + 1;
        }
        if(root.right == null) {
            return minDepth(root.left) + 1;
        }
        return 1 + Math.min(minDepth(root.left), minDepth(root.right));
    }
}