LeetCode sum-root-to-leaf-numbers

题目描述

Given a binary tree containing digits from0-9only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path1->2->3which represents the number123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

The root-to-leaf path1->2represents the number12.
The root-to-leaf path1->3represents the number13.

Return the sum = 12 + 13 =25.

解题思路：

• 初始sum = 0, 当前节点为root
• 判断rooot是否为叶子节点，如果是， sum += root.val; 否则，判断左子树是否为空？  如果不是，递归；判断右子树是否为空？ 如果不是，则递归

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int sum = 0;
  public int sumNumbers(TreeNode root) {
		if (root == null) return 0;
		dfs(root, 0);
		return sum;
	}
	
	public void dfs(TreeNode root, int pre) {
        if (root.left != null) {
            dfs(root.left, pre * 10 + root.val);
        }
		if (root.right != null) {
            dfs(root.right, pre * 10 + root.val);
        }
        if (root.left == null && root.right == null) {
            sum += pre * 10 + root.val;
            return;
        }
	}
}