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

Example:

3   / \  9  20    /  \   15   7There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24. 方法一：递归调用 时间复杂度：o（n）                   空间复杂度：o（1）

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {    public int sumOfLeftLeaves(TreeNode root) {        if(root==null) return 0;        if(isLeft(root.left)) return root.left.val+sumOfLeftLeaves(root.right); //找到左子叶之后继续找        return sumOfLeftLeaves(root.left)+sumOfLeftLeaves(root.right);     }    private boolean isLeft(TreeNode root){        if(root==null) return false;        return root.left==null&&root.right==null;  //子叶是没有左右孩子的结点    }}