Minimum Depth of Binary Tree

This problem gave me a hard time, basically what I missed is that the definition of the minimum depth. It should be “the shortest path from the root node down to the nearest leaf node.” Therefore if there is only a root and a node attached to it, the minimum depth will be 2 instead of 1.

The idea is recursion. Basically what I was thinking is do a tree traverse while keeping the minimum value, this is affected by the pocket algorithm I currently am studying. It turned out to be a much more effective way to do this.


 * 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) {
        return recursionStep(root);
    public static int recursionStep(TreeNode root){
        if(root == null) return 0;
        int left = recursionStep(root.left);
        int right = recursionStep(root.right);
        //to the nearest leaf node!!!
        if(left == 0) return right+1;
        if(right == 0) return left+1;
        return left < right? left+1 : right+1;

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