算法:Path Sum(路径总和)

2024年6月10日 80次阅读

说明

算法：Path Sum
LeetCode地址：https://leetcode.com/problems/path-sum/

题目：
Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

Note: A leaf is a node with no children.

Example:

Given the below binary tree and sum = 22,

      5
     / \
    4   8
   /   / \
  11  13  4
 /  \      \
7    2      1

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

解题思路

求各个路径的和，看是否有等于预设的值。 如果去掉跟节点的值 sum = sum - root.val , 问题就变成求左子树，或者右子树的和。则又是用到递归思想，子问题跟母问题的求解一样。

代码实现

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */
class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if (root == null) {
            return false;
        }
        if (root.left == null && root.right == null && root.val == sum) return true;
        return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);
    }
}

代码执行效率

Runtime: 0 ms, faster than 100.00% of Java online submissions for Path Sum.
Memory Usage: 38.8 MB, less than 19.75% of Java online submissions for Path Sum.

总结

求路径总和问题，实则考察递归的应用。

代码下载：
https://github.com/zgpeace/awesome-java-leetcode/blob/master/code/LeetCode/src/PathSum.java

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