LeetCode: Sum Root to Leaf Numbers 解题报告

Sum Root to Leaf Numbers

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.

SOLUTION 1:

使用递归完成。我们要做的是 把左右子树的path加起来。

base case: root是叶子节点时，直接计算path。我们用pre保留上一层计算出来的数字的值。那么到了当前根节点应该就是

把上一层的值*10再加上root的值。

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

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/tree/SumNumbers_1208_2014.java