LeetCode-Sum Root to Leaf Numbers

最新推荐文章于 2019-05-19 19:00:07 发布

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本文探讨了如何计算二叉树中从根节点到叶子节点的路径所组成的数字之和。通过递归算法，实现了一种有效的时间复杂度为O(n)的解决方案，其中n为树中的节点数量。

题目：https://oj.leetcode.com/problems/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.

源码：Java版本
算法分析：时间复杂度O(n)，空间复杂度O(logn)

/**
 * 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 sumNumbers(root,0);
    }
    
    private int sumNumbers(TreeNode node,int sum) {
        if(node==null) {
            return 0;
        }
        if(node.left==null && node.right==null) {
            return 10*sum+node.val;
        }
        return sumNumbers(node.left,10*sum+node.val)+
            sumNumbers(node.right,10*sum+node.val);
    }
}