Description,
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.

For 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.

The leetcode link

HashMap solution implemted in Java

Pre-order DFS tree traversal could be very strait-forward for solving this problem.

Time complexity: O(n)
RUntime: 1ms

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    /**
    * Returns boolean result whether any root-to-leaf path sums up to the "sum" given.
    * @param  TreeNode root of TreeNode type
    * @param  int int sum
    * @return boolean result
    */
    public boolean hasPathSum(TreeNode root, int sum) {
        if (root == null)   return false;
        if ((sum-root.val == 0) && (root.left == null) && (root.right == null)) {
            return true;
        }
        return hasPathSum(root.left, sum-root.val) || hasPathSum(root.right, sum-root.val);
    }
}

imgRuntime: 1ms