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.

/// 112. Path Sum
/// https://leetcode.com/problems/path-sum/description/
/// 時間複雜度: O(n), n為樹的節點個數
/// 空間複雜度: O(h), h為樹的高度
class Solution {

    // Definition for a binary tree node.
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    public boolean hasPathSum(TreeNode root, int sum) {

        if(root == null)
            return false;

        if(root.left == null && root.right == null)
            return sum == root.val;

        return hasPathSum(root.left, sum - root.val)
                || hasPathSum(root.right, sum - root.val);
    }
}
 

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1. 404sum of left leaves