# I am not sure where I did wrong. Need help!

• ``````    /**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {

private static int maximum = Integer.MIN_VALUE;
public static int maxPathSum(TreeNode root) {
maximum = Integer.MIN_VALUE;//the function would call many times, so need to reset maximum every time it called
maxPath(root);
return maximum;
}
private static int maxPath(TreeNode root) {
if(root == null)
return Integer.MIN_VALUE;
if( root.left== null && root.right == null){
if(root.val > maximum)
maximum = root.val;
return root.val;

}

int leftPath =  maxPath(root.left);
int rightPath = maxPath(root.right);
int sum = leftPath + rightPath + root.val;
int maxNum = findMax( root.val, leftPath+ root.val, rightPath+ root.val);
if ( Math.max(sum, maxNum) > maximum)
maximum = Math.max(sum, maxNum) ;
return maxNum;

}

private static int findMax ( int a, int b, int c){
int tmp1 = Math.max(a,b);

return Math.max(tmp1,c);
}
}
``````

I got wrong with { -2 1 } input the expected should be 1, but mine returns 21445223 ( can't remember the exact number but it is pretty large). I guess I need to reset my global variable every time call maxPathSum, but still doesn't work.

• The problem is negative number overflow.

``````int leftPath =  maxPath(root.left);
int rightPath = maxPath(root.right);
int sum = leftPath + rightPath + root.val;
``````

for the [-2, 1] case, leftPath is 1 and rightPath is Integer.MIN_VALUE. Thus:
sum = 1 + Integer.MIN_VALUE -2, which is Integer.MIN_VALUE - 1, overflow (or underflow?)

change:

``````if(root == null)
return Integer.MIN_VALUE;
``````

to:

``````if(root == null)
return 0;
``````

it will pass

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