Minimum Depth of Binary Tree

 

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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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/** * 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);    }    //这个题目和求最大(最小深度)不一样的是要走到叶子节点才算行,也就说要到了叶子节点才OK    //最開始的时候採取了(root == null)的推断，报错，是由于对根节点的处理中觉得是求最大的深度。而最小的深度实际是到了    //叶子节点之后才算行    int helper(TreeNode root) {        if (root.left == null && root.right == null) {            return 1;        }        if (root.left == null) {            return helper(root.right) + 1;        }             if (root.right == null) {            return helper(root.left) + 1;        }        else {        return Math.min(helper(root.left),helper(root.right)) + 1;        }    }}