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Verify Preorder Sequence in Binary Search Tree in C++
Suppose we have a sequence of numbers; we have to check whether it is the correct preorder traversal sequence of a binary search tree. We can assume that each number in the sequence is unique. Consider the following binary search tree −
So, if the input is like [5,2,1,3,6], then the output will be true
To solve this, we will follow these steps −
itr := -1
low := -infinity
for initialize i := 0, when i < size of preorder, update (increase i by 1), do −
x := preorder[i]
if x < low, then −
return false
while (itr >= 0 and preorder[itr] < x), do −
low := preorder[itr]
(decrease itr by 1)
(increase itr by 1)
preorder[itr] := x
return true
Example
Let us see the following implementation to get a better understanding −
#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
bool verifyPreorder(vector<int<& preorder) {
int itr = -1;
int low = INT_MIN;
for (int i = 0; i < preorder.size(); i++) {
int x = preorder[i];
if (x < low)
return false;
while (itr >= 0 && preorder[itr] < x) {
low = preorder[itr];
itr--;
}
itr++;
preorder[itr] = x;
}
return true;
}
};
main(){
Solution ob;
vector<int< v = {5,2,1,3,6};
cout << (ob.verifyPreorder(v));
}Input
{5,2,1,3,6}Output
1
Updated on: 18-Nov-2020
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