## Thursday, July 28, 2016

### Replace every element with the least greater element on its right - GeeksforGeeks

Replace every element with the least greater element on its right - GeeksforGeeks
Given an array of integers, replace every element with the least greater element on its right side in the array. If there are no greater element on right side, replace it with -1.

A tricky solution would be to use Binary Search Trees. We start scanning the array from right to left and insert each element into the BST. For each inserted element, we replace it in the array by its inorder successor in BST. If the element inserted is the maximum so far (i.e. its inorder successor doesn’t exists), we replace it by -1.
`/* A utility function to insert a new node with`
`   ``given data in BST and find its successor */`
`void` `insert(Node*& node, ``int` `data, Node*& succ)`
`{`
`    ``/* If the tree is empty, return a new node */`
`    ``if` `(node == NULL)`
`        ``node = newNode(data);`

`    ``// If key is smaller than root's key, go to left`
`    ``// subtree and set successor as current node`
`    ``if` `(data < node->data)`
`    ``{`
`        ``succ = node;`
`        ``insert(node->left, data, succ);`
`    ``}`

`    ``// go to right subtree`
`    ``else` `if` `(data > node->data)`
`        ``insert(node->right, data, succ);`
`}`

`// Function to replace every element with the`
`// least greater element on its right`
`void` `replace(``int` `arr[], ``int` `n)`
`{`
`    ``Node* root = NULL;`

`    ``// start from right to left`
`    ``for` `(``int` `i = n - 1; i >= 0; i--)`
`    ``{`
`        ``Node* succ = NULL;`

`        ``// insert current element into BST and`
`        ``// find its inorder successor`
`        ``insert(root, arr[i], succ);`

`        ``// replace element by its inorder`
`        ``// successor in BST`
`        ``if` `(succ)`
`            ``arr[i] = succ->data;`
`        ``else`    `// No inorder successor`
`            ``arr[i] = -1;`
`    ``}`
`}`

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