前序遍历解决路径总和问题

    public ArrayList<ArrayList<Integer>> pathSum (TreeNode root, int sum) {
        // 前序遍历方法

        preOrder(root, sum, 0);
        return res;
    }

    public void preOrder(TreeNode root, int sum, int cur){
        if(root == null) return;

        temp.add(root.val);
        cur +=root.val;
        if(root.left == null && root.right == null && cur == sum){
            res.add(new ArrayList<Integer>(temp));
        }else{
            preOrder(root.left, sum, cur);
            preOrder(root.right, sum, cur);
        }


        temp.remove(temp.size() - 1);
    }

二叉树中最大路径总和

class Solution {
    int ans = Integer.MIN_VALUE;
    public int maxPathSum(TreeNode root) {
       postorder(root);
       return ans;
    }
    public int postorder(TreeNode root){
        if(root == null)
            return 0;
        int left = Math.max(0,  postorder(root.left));
        int right = Math.max(0, postorder(root.right));

        ans = Math.max(ans, left + right + root.val);
        return Math.max(left, right) + root.val;
    }
}