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.

 思路:到leaf的时候判断是不是left leaf。用一个boolean function来记录搜索的是左枝还是右枝。

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