# Find root of the tree where children id sum for every node is given

Hello Everyone,

Consider a binary tree whose nodes have ids from 1 to n where n is number of nodes in the tree. The tree is given as a collection of n pairs, where every pair represents node id and sum of children ids.

At first sight this question appears to be a typical question of tree data structure but it
can be solved as follows.

Every node id appears in children sum except root. So if we do sum of all ids and subtract it from sum of all children sums, we get root.

`// Find root of tree where children`

`// sum for every node id is given.`

`#include<bits/stdc++.h>`

`using` `namespace` `std;`

` `

`int` `findRoot(pair<` `int` `, ` `int` `> arr[], ` `int` `n)`

`{`

` ` `// Every node appears once as an id, and`

` ` `// every node except for the root appears`

` ` `// once in a sum. So if we subtract all`

` ` `// the sums from all the ids, we're left`

` ` `// with the root id.`

` ` `int` `root = 0;`

` ` `for` `(` `int` `i=0; i<n; i++)`

` ` `root += (arr[i].first - arr[i].second);`

` `

` ` `return` `root;`

`}`

` `

`// Driver code`

`int` `main()`

`{`

` ` `pair<` `int` `, ` `int` `> arr[] = {{1, 5}, {2, 0},`

` ` `{3, 0}, {4, 0}, {5, 5}, {6, 5}};`

` ` `int` `n = ` `sizeof` `(arr)/` `sizeof` `(arr[0]);`

` ` `printf` `(` `"%d\n"` `, findRoot(arr, n));`

` ` `return` `0;`

`}`

Output:

6

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Thankyou.