LeetCode-Minimum Depth of Binary Tree

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Description：
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.

题意：求一颗二叉树中叶子节点到根节点的最短路径的深度；

解法：求一颗树的最小深度，也就是递归的求左右子树的最小深度；在递归求解的时候，我们需要特别判断的是当前节点是否是叶子节点，因此我们对一个节点的左右子节点中如果是一个空节点，返回一个较大值，则当前节点的左右子树最小深度就不会去计算那个子树中的空节点；

Java

/**
 * 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) {
        return root == null ? 0 : getMinDepth(root);
    }
    private int getMinDepth(TreeNode root) {
        if (root == null) return Integer.MAX_VALUE;
        if (root.left == null && root.right == null) return 1;
        int left = getMinDepth(root.left);
        int right = getMinDepth(root.right);
        return left < right ? left + 1 : right + 1;
    }
}