August 08, 2019

124. Binary Tree Maximum Path Sum

Time O(n), Space O(h)

Pay attention:

Initiate max_sum as the smallest possible integer
for helper method:
- Base case: if node is null, the max gain is 0.
- What to do in current layer: Update max_sum
- What to return to parent node:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    private int max = Integer.MIN_VALUE; // important to initialize max to MIN_VALUE
    
    public int maxPathSum(TreeNode root) {
        maxPathSumHelper(root);
        return max;
    }
    
    private int maxPathSumHelper(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int left = Math.max(0, maxPathSumHelper(root.left)); // 考虑负数情况
        int right = Math.max(0, maxPathSumHelper(root.right)); // 考虑负数情况
        max = Math.max(max, left + right + root.val); // 更新 global value
        return Math.max(left, right) + root.val; // 当前层返值
    }
}