sumNumbers

sumNumbers

Author: yuzhangcmu

tree/SumNumbers.java

@@ -0,0 +1,67 @@
+package Algorithms.tree;
+
+import java.util.ArrayList;
+
+/*
+ * 
+ * Sum Root to Leaf Numbers Total Accepted: 23940 Total Submissions: 80436 My Submissions
+Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.
+
+An example is the root-to-leaf path 1->2->3 which represents the number 123.
+
+Find the total sum of all root-to-leaf numbers.
+
+For example,
+
+    1
+   / \
+  2   3
+The root-to-leaf path 1->2 represents the number 12.
+The root-to-leaf path 1->3 represents the number 13.
+
+Return the sum = 12 + 13 = 25.
+ * */
+
+public class SumNumbers {
+    public int sumNumbers(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        
+        ArrayList<Integer> ret = new ArrayList<Integer>();
+        
+        // 存储从根节点到当前节点的路径上的数字
+        ArrayList<Integer> path =  new ArrayList<Integer>();
+        
+        dfs(root, path, ret);
+        int sum = 0;
+        for (int n: ret) {
+            sum += n;
+        }
+        
+        return sum;
+    }
+    
+    public void dfs(TreeNode root, ArrayList<Integer> path, ArrayList<Integer> ret) {
+        if (root == null) {
+            return;
+        }
+
+        path.add(root.val);
+
+        if (root.left == null && root.right == null) {
+            int num = 0;
+            for (int n: path) {
+                num = num * 10 + n;
+            }
+            ret.add(num);
+        } else {
+            // 向左右子树递归
+            dfs(root.left, path, ret);
+            dfs(root.right, path, ret);    
+        }
+
+        // 一定要记得回溯，也就是说递归不能修改Path本身，否则以上向左右子树分别递归时 path就会被改。 
+        path.remove(path.size() - 1);
+    }
+}