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leetcode 124. binary tree maximum path sum

admin 11月 13, 2020 0

124. Binary Tree Maximum Path Sum

Difficulty:: Hard

Given a non-empty binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root.

Example 1:

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Input: [1,2,3]

       1
      / 
     2   3

Output: 6

Example 2:

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Input: [-10,9,20,null,null,15,7]

    -10
     / 
   9   20
      /   
     15    7

Output: 42

Solution

Language: Java

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 * Definition for a binary tree node.
 * public class TreeNode {
 *       int val;
 *       TreeNode left;
 *       TreeNode right;
 *       TreeNode(int x) { val = x; }
 * }
 */
class  {
      int max;
      public int maxPathSum(TreeNode root) {
            max = Integer.MIN_VALUE;
            if (root == null) {
                  return 0;
           }
            helper(root);
            return max;
     }
      
      // return max
      private int helper(TreeNode root) {
            if (root == null) {
                  return 0;
           }
            int leftMax = Math.max(helper(root.left), 0);
            int rightMax = Math.max(helper(root.right), 0);
            int totalMax = leftMax + rightMax + root.val;
            max = Math.max(totalMax, max);
            return root.val + Math.max(leftMax, rightMax);
     }
}
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