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.

一刷
题解：
把tree的问题分解成left和right的subtree问题。用DFS求解

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        return sum(root, false);
    }
    
    private int sum(TreeNode root, boolean isLeft){
        if(root == null) return 0;
        if(root.left==null && root.right==null && isLeft) {
            return root.val;
        }
        
        int left = sum(root.left, true);
        int right = sum(root.right, false);
        
        return left + right;
    }
}