easyDSA

How do you check if a root-to-leaf path sums to a target?

Q: How do you check if a root-to-leaf path sums to a target?

**Intuition.** Carry the target *down* the tree, **subtracting** each node's value as you descend. The question "does some root-to-leaf path sum to T ?" becomes "at this child, does some path sum to T − node.val ?" — a smaller version of the same problem. Success means hitting a **leaf** where the remaining target equals the leaf's value. target 20: subtract down → leaf reaches 0 along 5→4→11 <line x1="170" y1="106" x2="130" y2="146" s

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Explain path sum.

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DFS subtracting the target

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class TreeNode:
    def __init__(self, val, left=None, right=None):
        self.val, self.left, self.right = val, left, right

def has_path_sum(root, target):
    if not root:
        return False
    if not root.left and not root.right:    # leaf
        return target == root.val
    rest = target - root.val
    return has_path_sum(root.left, rest) or has_path_sum(root.right, rest)


# --- demo ---
root = TreeNode(5, TreeNode(4, TreeNode(11)), TreeNode(8))
print(has_path_sum(root, 20))   # True   (5 -> 4 -> 11)
print(has_path_sum(root, 21))   # False

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