Merge pull request #105 from Joshua-Lu/patch-22

添加 0111.二叉树的最小深度 Java版本 递归与迭代

problems/0111.二叉树的最小深度.md

@@ -194,17 +194,61 @@ public:
 
 
 Java：
-```java
-class Solution {
-    public int minDepth(TreeNode root) {
-        if (root == null) return 0;
-        int leftDeep = minDepth(root.left);
-        int rightDeep = minDepth(root.right);
-        
-        if (root.left == null && root.right != null) return 1+rightDeep;
-        if (root.left != null && root.right == null) return 1+leftDeep;
 
-        return Math.min(leftDeep,rightDeep)+1;
+```Java
+class Solution {
+    /**
+     * 递归法，相比求MaxDepth要复杂点
+     * 因为最小深度是从根节点到最近**叶子节点**的最短路径上的节点数量
+     */
+    public int minDepth(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        int leftDepth = minDepth(root.left);
+        int rightDepth = minDepth(root.right);
+        if (root.left == null) {
+            return rightDepth + 1;
+        }
+        if (root.right == null) {
+            return leftDepth + 1;
+        }
+        // 左右结点都不为null
+        return Math.min(leftDepth, rightDepth) + 1;
+    }
+}
+```
+
+```Java
+class Solution {
+   /**
+     * 迭代法，层序遍历
+     */
+    public int minDepth(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        Deque<TreeNode> deque = new LinkedList<>();
+        deque.offer(root);
+        int depth = 0;
+        while (!deque.isEmpty()) {
+            int size = deque.size();
+            depth++;
+            for (int i = 0; i < size; i++) {
+                TreeNode poll = deque.poll();
+                if (poll.left == null && poll.right == null) {
+                    // 是叶子结点，直接返回depth，因为从上往下遍历，所以该值就是最小值
+                    return depth;
+                }
+                if (poll.left != null) {
+                    deque.offer(poll.left);
+                }
+                if (poll.right != null) {
+                    deque.offer(poll.right);
+                }
+            }
+        }
+        return depth;
     }
 }
 ```
@@ -264,4 +308,4 @@ Go：
 * 作者微信：[程序员Carl](https://mp.weixin.qq.com/s/b66DFkOp8OOxdZC_xLZxfw)
 * B站视频：[代码随想录](https://space.bilibili.com/525438321)
 * 知识星球：[代码随想录](https://mp.weixin.qq.com/s/QVF6upVMSbgvZy8lHZS3CQ)
-<div align="center"><img src=../pics/公众号.png width=450 alt=> </img></div>
+<div align="center"><img src=../pics/公众号.png width=450 alt=> </img></div>