SumOfLeftLeaves.java

package binary_tree;

/**
 * @Author: Wenhang Chen
 * @Description:计算给定二叉树的所有左叶子之和。 示例：
 * <p>
 * 3
 * / \
 * 9  20
 * /  \
 * 15   7
 * <p>
 * 在这个二叉树中，有两个左叶子，分别是 9 和 15，所以返回 24
 * @Date: Created in 10:16 3/2/2020
 * @Modified by:
 */
public class SumOfLeftLeaves {
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }

    public int sumOfLeftLeaves(TreeNode root) {
        return sumOfLeftLeavesHelper(root, false);
    }

    // 先序遍历求所有左叶子节点值之和
    public int sumOfLeftLeavesHelper(TreeNode root, boolean flag) {
        if (root == null) {
            return 0;
        }
        int leave = 0;
        // 左叶子节点
        if (flag && root.left == null && root.right == null) {
            leave = root.val;
        }
        int left = sumOfLeftLeavesHelper(root.left, true);
        int right = sumOfLeftLeavesHelper(root.right, false);
        return left + right + leave;
    }

}