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Example tree trace
Tree: root −10, left 9, right 20 (left 15, right 7). DFS leaf 15: returns 15. DFS leaf 7: returns 7. DFS node 20: leftMax =15, rightMax =7. Updates maxPath =20+15+7=42.
Returns 20+15=35. DFS leaf 9: returns 9. DFS root −10: leftMax =9, rightMax =35. Updates maxPath =max(42,−10+9+35)=max(42,34)=42. Returns −10+35=25. Answer: 42.
