404. Sum of Left Leaves

Posted 2020-08-11 咖啡中不塌缩的方糖

Find the sum of all left leaves in a given binary tree.

Example:

    3
   /   9  20
    /     15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.


Recusive:

 public int SumOfLeftLeaves(TreeNode root) {
        return DFS(root,false);
    }
    
    public int DFS(TreeNode root, bool left)
    {
        if(root == null) return 0;
        if(root.left==null && root.right==null ) return left?root.val:0;
        return  DFS(root.left,true)+DFS(root.right,false);
    }

Iteration:

public int SumOfLeftLeaves(TreeNode root) {
        if(root == null) return 0;
        var stack = new Stack<TreeNode>();
        int sum =0;
        stack.Push(root);
        while(stack.Count()>0)
        {
            var a = stack.Pop();
            if(a.left != null)
            {
                 if(a.left.left ==null && a.left.right==null) sum += a.left.val;
                 else stack.Push(a.left);
            }
            if(a.right != null)
            stack.Push(a.right);
        }
        return sum;
    }