Minimum Depth of Binary Tree | Leetcode

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.

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Solution: A tricky question. The minimum depth in this question must be traced down to a leaf node. For the non-leaf node, we will never take its depth as the answer.

For example, this tree has a minimum depth of 3 even though the 2 has no right child, which normally gives a minimum depth of 2.

             1
            / \
           2   3
          /     \
         4       5

For the algorithm, we ignore the missing left or right child if the other child (its sibling) is present.

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