[leet code] 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.

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Analysis:

1. Recursive approach, each recessive call focuses on one node

2. If leaf node reached, return depth

3. If current node has left or right child or both, return the minimum depth among left subtree and right subtree.

/**
 * Definition for binary tree
 * 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;
        return helper(root, 0, Integer.MAX_VALUE);
    }
    
    public int helper(TreeNode node, int depth, int minDepth){
        if(node == null) return minDepth;
        
        depth++;
        
        // current node is leaf
        if(node.left == null && node.right == null) return depth;
        
        // current node not leaf
        minDepth = Math.min(helper(node.left, depth, minDepth), helper(node.right, depth, minDepth));
        return minDepth;
    }
}