A single line implementation of checking if a tree is sorted.

IsTheTreeSorted.c

#include <stdio.h>
#include <limits.h>

// A macro to return a string of word true or false based on the evaluation given.
#define boolToString(_bool_) ((_bool_) ? "true" : "false")

// A very simple tree
typedef struct _TreeNode
{
  struct _TreeNode *left;
  struct _TreeNode *right;

  int value;
} TreeNode;

// When calling this function, you should send a reference to an integer who's value is INT_MIN. If you send an invalid pointer a crash should be expected.
int isTreeSorted(TreeNode *node, int *previous)
{
  return node == NULL || (isTreeSorted(node->left, previous) && (*previous <= node->value) && ((*previous = node->value) || 1) && isTreeSorted(node->right, previous));
}

int main(void)
{
  int desiredAnswer = 1;

 // Build the tree
  TreeNode l11 = {NULL, NULL, 1};
  TreeNode l12 = {NULL, NULL, desiredAnswer ? 7 : 15};
  TreeNode l1 = {&l11, &l12, 5};
  TreeNode r1 = {NULL, NULL, 20};
  TreeNode tree = {&l1, &r1, 10};

 // Grab the answer
  int previousValue = INT_MIN;
  int answer = isTreeSorted(&tree, &previousValue) == 1;

 // Print the result
  printf("sorted = %s, correct = %s\n", boolToString(answer), boolToString(answer == desiredAnswer));

  return 0;
}