[Leet Code]Path Sum

public class Solution {
    private int currSum = 0;
    
    public boolean hasPathSum(TreeNode root, int sum) {
        if (null == root)
            return false;
        
        currSum = 0;
        return hasPathSumCore(root, sum);
    }
    
    public boolean hasPathSumCore(TreeNode root, int sum) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (null == root)
            return true;
        
        currSum += root.val;

        boolean leftHas = false;
        boolean rightHas = false;
        
        if (null == root.left && null == root.right) {
            if (currSum == sum) {
                currSum -= root.val;
                return true;
            }
            else {
                currSum -= root.val;
                return false;
            }
        }
        else if (null == root.left && null != root.right) {
            rightHas = hasPathSumCore(root.right, sum);
            currSum -= root.val;
            return rightHas;
        }
        else if (null == root.right && null != root.left) {
            leftHas = hasPathSumCore(root.left, sum);
            currSum -= root.val;
            return leftHas;
        }
        else {
            leftHas = hasPathSumCore(root.left, sum);
            rightHas = hasPathSumCore(root.right, sum);
            currSum -= root.val;
            return leftHas || rightHas;
        }
    }
}