求二叉树的最大路径和

//二叉树节点的定义
class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode(int x) { val = x; }
  }

public class Solution {

    //方法一:递归解法，时间复杂度过高
    public int maxPathSum2(TreeNode root) {
      if(root==null)
        return 0;

      int count=Math.max(maxPathSum(root.left),maxPathSum(root.right));
      int tem=Math.max(count,count+root.val);
      return Math.max(tem,maxPathSum(root.left)+maxPathSum(root.right)+root.val);

    }
    int maxValue;
    public int maxPathSum(TreeNode root) {
        maxValue=Integer.MIN_VALUE;
        maxPath(root);

        return maxValue;
    }

    //方法二:递归解法的优化
    public int maxPath(TreeNode node) {
      if(node==null)
         return 0;
      int left=Math.max(0,maxPath(node.left));
      int right=Math.max(0,maxPath(node.right));
      maxValue=Math.max(maxValue,left+right+node.val);

      return Math.max(left,right)+node.val;
    }

    public static void main(String[]args){
      // System.out.println("Hello");
      TreeNode root=new TreeNode(-1);
      root.left=new TreeNode(2);
      root.right=new TreeNode(3);
      root.left.left=new TreeNode(4);
      Solution s=new Solution();
      System.out.println(s.maxPathSum(root));

    }
}