Merge pull request #2173 from lifejwang11/master

修改java版本解法的位置，适合图的解法放在第一个，第二个解法（难理解版本）放在其后，新增java版本的迭代法

problems/0077.组合.md

@@ -343,8 +343,32 @@ public:
 ## 其他语言版本
 
 
-### Java
 
+### Java：
+未剪枝优化
+```java
+class Solution {
+    List<List<Integer>> result= new ArrayList<>();
+    LinkedList<Integer> path = new LinkedList<>();
+    public List<List<Integer>> combine(int n, int k) {
+        backtracking(n,k,1);
+        return result;
+    }
+
+    public void backtracking(int n,int k,int startIndex){
+        if (path.size() == k){
+            result.add(new ArrayList<>(path));
+            return;
+        }
+        for (int i =startIndex;i<=n;i++){
+            path.add(i);
+            backtracking(n,k,i+1);
+            path.removeLast();
+        }
+    }
+}
+```
+剪枝优化：
 ```java
 class Solution {
     List<List<Integer>> result = new ArrayList<>();

problems/0450.删除二叉搜索树中的节点.md

@@ -268,6 +268,34 @@ public:
 
 
 ### Java 
+```java
+// 解法1(最好理解的版本)
+class Solution {
+  public TreeNode deleteNode(TreeNode root, int key) {
+    if (root == null) return root;
+    if (root.val == key) {
+      if (root.left == null) {
+        return root.right;
+      } else if (root.right == null) {
+        return root.left;
+      } else {
+        TreeNode cur = root.right;
+        while (cur.left != null) {
+          cur = cur.left;
+        }
+        cur.left = root.left;
+        root = root.right;
+        return root;
+      }
+    }
+    if (root.val > key) root.left = deleteNode(root.left, key);
+    if (root.val < key) root.right = deleteNode(root.right, key);
+    return root;
+  }
+}
+```
+
+
 ```java
 class Solution {
     public TreeNode deleteNode(TreeNode root, int key) {
@@ -296,34 +324,59 @@ class Solution {
     }
 }
 ```
+递归法
 ```java
-// 解法2
 class Solution {
   public TreeNode deleteNode(TreeNode root, int key) {
-    if (root == null) return root;
+    if (root == null){
-    if (root.val == key) {
+      return null;
-      if (root.left == null) {
+    }
-        return root.right;
+    //寻找对应的对应的前面的节点，以及他的前一个节点
-      } else if (root.right == null) {
+    TreeNode cur = root;
-        return root.left;
+    TreeNode pre = null;
-      } else {
+    while (cur != null){
-        TreeNode cur = root.right;
+      if (cur.val < key){
-        while (cur.left != null) {
+        pre = cur;
+        cur = cur.right;
+      } else if (cur.val > key) {
+        pre = cur;
+        cur = cur.left;
+      }else {
+        break;
+      }
+    }
+    if (pre == null){
+      return deleteOneNode(cur);
+    }
+    if (pre.left !=null && pre.left.val == key){
+      pre.left = deleteOneNode(cur);
+    }
+    if (pre.right !=null && pre.right.val == key){
+      pre.right = deleteOneNode(cur);
+    }
+    return root;
+  }
+
+  public TreeNode deleteOneNode(TreeNode node){
+    if (node == null){
+      return null;
+    }
+    if (node.right == null){
+      return node.left;
+    }
+    TreeNode cur = node.right;
+    while (cur.left !=null){
       cur = cur.left;
     }
-        cur.left = root.left;
+    cur.left = node.left;
-        root = root.right;
+    return node.right;
-        return root;
-      }
-    }
-    if (root.val > key) root.left = deleteNode(root.left, key);
-    if (root.val < key) root.right = deleteNode(root.right, key);
-    return root;
   }
 }
 ```
 
+
 ### Python 
+
 递归法（版本一）
 ```python
 class Solution: