111. 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.

这里用递归解法，分了5种返回情况，详见代码，代码如下：

/** * 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 && root.right != null) {            return minDepth(root.right) + 1;        }        if (root.left != null && root.right == null) {            return minDepth(root.left) + 1;        }        return Math.min(minDepth(root.left), minDepth(root.right)) + 1;    }}

也可以分别计算left和right，如果其中一方为0，返回left+right+1，如果都不为0，返回较小值+1，代码如下：

public class Solution {    public int minDepth(TreeNode root) {        if(root == null) return 0;        int left = minDepth(root.left);        int right = minDepth(root.right);        return (left == 0 || right == 0) ? left + right + 1: Math.min(left,right) + 1;           }}

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