Path Sum

[4, 2, 7, 1, 3, 6, 9]
target = 17 

[4, 2, 7, 1, 3, 6, 9]
target = 13

// class TreeNode {
//     int val;
//     TreeNode left;
//     TreeNode right;
//     TreeNode() {}
//     TreeNode(int val) { this.val = val; }
//     TreeNode(int val, TreeNode left, TreeNode right) {
//         this.val = val;
//         this.left = left;
//         this.right = right;

Explanation

We can solve this problem using the framework established in the Overview section.

Return Values

If I'm at a node in the tree, what values do I need from my child to tell if there's a path from the root to a leaf that sums to the target?

In order for there to be a path from the root to a leaf that sums to the target, there must be a path from either the left or right child to a leaf that sums to target - node.val.

This tells us that each recursive call should return True if there's a path from the current node to a leaf that sums to the current target, and False otherwise. In the body, we'll make recursive calls to the current node's left and right children, passing in target - node.val for the current target. If either of those calls returns True, then we should return True to the parent.

Base Case If our current node is None, then our subtree is empty and there's no path from the current node to a leaf that sums to the target.

If our current node is a leaf node, then we check if the target is equal to the leaf node's value. If it is, then there's a path from the current node to a leaf that sums to the target.

Helper Functions We need to pass the current target down from parent to children nodes, but this is included in the function signature of the main function, so we don't need to introduce a helper function.

Global Variables The return value of each recursive call matches the answer to the problem, so we don't need to use any global variables.

Solution

class Solution {
    public boolean pathSum(TreeNode root, int target) {
        if (root == null) {
            return false;
        }
       
        // if we reach a leaf node, check if the target is equal to the leaf node's value
        if (root.left == null && root.right == null) {
            return target == root.val;
        }
        target -= root.val;
        
        // check if there's a path from the current node to a leaf that sums to target
        return pathSum(root.left, target) || pathSum(root.right, target);
    }
}

Animated Solution

public boolean hasPathSum(TreeNode root, int target) {
    if (root == null) {
        return false;
    }
    
    if (root.left == null && root.right == null) {
        return target == root.val;
    }
    
    boolean left = hasPathSum(root.left, target - root.val);
    boolean right = hasPathSum(root.right, target - root.val);
    return left || right;
}

target: 13