GATE CS 2023 Set 1 — Question 47

MCQ+2 / -0.67MediumFunctions & RecursionC ProgrammingProgramming & Data StructuresTree TraversalsTrees

Programming & Data Structures → C Programming → Functions & Recursion

Last updated 5 September 2026

Question

Consider the C function foo and the binary tree shown. typedef struct node { int val; struct node *left, *right; } node; int foo(node *p) { int retval; if (p == NULL) return 0; else { retval = p->val + foo(p->left) + foo(p->right); printf("%d ", retval); return retval; } } When foo is called with a pointer to the root node of the given binary tree, what will it print?

Correct answer (C) 3 8 16 13 24 50 Solution The function foo calculates the sum of all node values in the subtree rooted at p . It uses a post-order traversal (Left, Right, Root) because it recursively calls itself on the left and right children before calculating retval and printing it. Let's trace the execution: 1. foo(3) : retval = 3 + 0 + 0 = 3 . Prints "3 ". Returns 3. 2. foo(8) : retval = 8 + 0 + 0 = 8 . Prints "8 ". Returns 8. 3. foo(5) : retval = 5 + foo(3) + foo(8) = 5 + 3 + 8 = 16 . Prints "16 ". Returns 16. 4. foo(13) : retval = 13 + 0 + 0 = 13 . Prints "13 ". Returns 13. 5. foo(11) : retval = 11 + foo(NULL) + foo(13) = 11 + 0 + 13 = 24 . Prints "24 ". Returns 24. 6. foo(10) : retval = 10 + foo(5) + foo(11) = 10 + 16 + 24 = 50 . Prints "50 ". Returns 50. The sequence of printed values is: 3 8 16 13 24 50. Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (C) 3 8 16 13 24 50 Solution The function foo calculates the sum of all node values in the subtree rooted at p . It uses a post-order traversal (Left, Right, Root) because it recursively calls itself on the left and right children before calculating retval and printing it. Let's trace the execution: 1. foo(3) : retval = 3 + 0 + 0 = 3 . Prints "3 ". Returns 3. 2. foo(8) : retval = 8 + 0 + 0 = 8 . Prints "8 ". Returns 8. 3. foo(5) : retval = 5 + foo(3) + foo(8) = 5 + 3 + 8 = 16 . Prints "16 ". Returns 16. 4. foo(13) : retval = 13 + 0 + 0 = 13 . Prints "13 ". Returns 13. 5. foo(11) : retval = 11 + foo(NULL) + foo(13) = 11 + 0 + 13 = 24 . Prints "24 ". Returns 24. 6. foo(10) : retval = 10 + foo(5) + foo(11) = 10 + 16 + 24 = 50 . Prints "50 ". Returns 50. The sequence of printed values is: 3 8 16 13 24 50.

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