404. Sum of Left Leaves

题目404. Sum of Left Leaves

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.

1,深搜

public class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        return dfs(root) ;
    }
    
    private int dfs(TreeNode root){
        if(root == null){
            return 0;
        }
        
        if(root.left == null && root.right == null){
            return 0;
        }
        
        if(root.left != null && root.left.left == null && root.left.right == null){
            return root.left.val+ dfs(root.right);
        }
        return dfs(root.left) + dfs(root.right);
    }
}