124. Binary Tree Maximum Path Sum.md

124. Binary Tree Maximum Path Sum

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leetcode-cn Daily Challenge on June 21, 2020

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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:

Input: [1,2,3]

       1
      / \
     2   3

Output: 6

Example 2:

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

   -10
   / \
  9  20
    /  \
   15   7

Output: 42

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Solution

• mine
  • Java
    • DFS Runtime: 0 ms, faster than 100.00%, Memory Usage: 41.1 MB, less than 88.47% of Java online submissions

      // O(N)time O(D)space
      // N = node.size
      // D = root's depth
      public int res = Integer.MIN_VALUE;
      public int maxPathSum(TreeNode root) {
          dfs(root);
          return res;
      }
      
      public int dfs(TreeNode node){
          if(node == null){
              return 0;
          }
          int left = Math.max(dfs(node.left), 0);
          int right = Math.max(dfs(node.right), 0);
          int count = left + right + node.val;
          res = Math.max(res, count);
          return node.val + Math.max(right, left);
      }
      

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