Path Sum

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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.

public class Solution {
    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);
    }
}

Binary Tree Paths

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Given a binary tree, return all root-to-leaf paths.

public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> ans = new ArrayList<>();
        if (root != null) dfs(root, "" + root.val, ans);
        return ans;
    }
    public void dfs(TreeNode node, String soFar, List<String> ans) {
        if (node.left == null && node.right == null) ans.add(soFar);
        if (node.left != null) dfs(node.left, soFar + "->" + node.left.val , ans);
        if (node.right != null) dfs(node.right, soFar + "->" + node.right.val, ans);
    }
}