Leetcode(129) Sum Root to Leaf Numbers

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Description

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.

Note: A leaf is a node with no children.

Example:

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9
Input: [1,2,3]
    1
   / \
  2   3
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.

Example 2:

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9
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12
Input: [4,9,0,5,1]
    4
   / \
  9   0
 / \
5   1
Output: 1026
Explanation:
The root-to-leaf path 4->9->5 represents the number 495.
The root-to-leaf path 4->9->1 represents the number 491.
The root-to-leaf path 4->0 represents the number 40.
Therefore, sum = 495 + 491 + 40 = 1026.

解法

DFS遍历每一条自顶向下的路径然后累加求和即可~

具体代码如下：

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class Solution {
    List<Integer> res = new ArrayList<>();

    public int sumNumbers(TreeNode root) {
        if (root == null) {
            return 0;
        }
        dfs(root, root.val);
        int result = 0;
        for (Integer x : res) {
            result += x;
        }
        return result;
    }

    public void dfs(TreeNode root, int pathSum) {
        if (root.left == null && root.right == null) {
            res.add(pathSum);
        } else if (root.left == null) {
            dfs(root.right, pathSum * 10 + root.right.val);
        } else if (root.right == null) {
            dfs(root.left, pathSum * 10 + root.left.val);
        } else {
            dfs(root.right, pathSum * 10 + root.right.val);
            dfs(root.left, pathSum * 10 + root.left.val);
        }
    }
}