[129. Sum Root to Leaf Numbers] #leetcode

129_0829.java

class Solution {
    private int res = 0;
    
    public int sumNumbers(TreeNode root) {
        if ( root == null ) return 0;
        dfs(root, 0);
        return res;
    }
    
    public void dfs(TreeNode node, int sum) {
        if ( node.left == null && node.right == null ) {
            res += sum*10 + node.val;
        }
        
        if ( node.left != null ) dfs(node.left, sum*10+node.val);
        if ( node.right != null ) dfs(node.right, sum*10+node.val);
        
    }
}