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# Sink All Zero

## Question

Given a binary tree containing only 0 and 1, sink all 0 to the bottom of the binary tree. ('Sink' means for all the node with value 0, its child node should all be 0.)

## Solution

```java
public TreeNode sinkAllZero(TreeNode root) {
    if(root==null) return root;    
    sinkAllZero(root.left);
    sinkAllZero(root.right);
    sink(root);
    return root;
}


private void sink(TreeNode root) {
    // in this time, root.val = 0.
    if(root.val==1) return;
    if((root.left==null || root.left.val==0) && (root.right==null || root.right.val==0)) return;
    if(root.left!=null && root.left.val==1) {
        root.left.val = 0;
        root.val = 1;
        sink(root.left);
    }else {
        root.right.val = 0;
        root.val = 1;
        sink(root.right);
    }
}
```

"Any fool can write code that a computer can understand. Good programmers write code that humans can understand." --- Martin Fowler