SumOfLeftLeaves.java

class SumOfLeftLeaves {
    
    private int leavesSum;
    
    public int sumOfLeftLeaves(TreeNode root) {
        leavesSum = 0;
        dfs(root, false);
        return leavesSum;
    }
    
    private void dfs(TreeNode node, boolean left) {
        if (node == null) {
            return;
        }
        if (node.left == null && node.right == null && left) {
            leavesSum += node.val;
        } else {
            dfs(node.left, true);
            dfs(node.right, false);
        }
    }
    
}