404. Sum of Left Leaves

Leetcode Tree

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.

分析

如果是求所有叶子结点的和会比较简单，为了区分左叶子结点和右叶子节点，特别的，在参数列表上增加了isLeft参数。

private static int sum;
public int sumOfLeftLeaves(TreeNode root) {
    sum = 0;
    sumOfLeftLeavesHelper(root, false); 
    return sum;
}
private void sumOfLeftLeavesHelper(TreeNode root, boolean isLeft){
    if (root == null) return;
    if (root.left == null && root.right == null && isLeft) sum += root.val;
    sumOfLeftLeavesHelper(root.left, true);
    sumOfLeftLeavesHelper(root.right, false);
}