diff --git a/code/lc968.java b/code/lc968.java
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+package code;
+/*
+ * 968. Binary Tree Cameras
+ * 题意：在树上放置camera, 覆盖整棵树，一个camera可以监视自身，父节点和孩子节点，问最少camera数量
+ * 难度：Hard
+ * 分类：Dynamic Programming, Tree, Depth-first Search
+ * 思路：贪心算法，尽量让父节点放置
+ * Tips：
+ */
+public class lc968 {
+    public class TreeNode {
+        int val;
+        TreeNode left;
+        TreeNode right;
+        TreeNode(int x) { val = x; }
+    }
+    int res;
+    public int minCameraCover(TreeNode root) {
+        if(root==null) return 0;
+        int flag = dfs(root);
+        if(flag==3) return res+1; //如果是3，代表root没被覆盖
+        return res;
+    }
+
+    public int dfs(TreeNode tn){
+        // 1代表被覆盖了，但该点没有camera。left,right都不为3的话，贪心算法，父节点不需要放置
+        // 2代表被覆盖了，该点有camera，父节点不需要放置
+        // 3代表没有被覆盖，所以父节点要放置
+        if(tn==null) return 1;
+        int left = dfs(tn.left);
+        int right = dfs(tn.right);
+        if( left==3 || right==3){
+            res+=1; //在该节点放置
+            return 2;
+        }
+        if( left==2 || right==2 ) return 1;
+        return 3;
+    }
+}