https://leetcode.com/problems/count-univalue-subtrees/description/
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这道题需要子树所有的VALUE都一致，我们就需要对子树去选队长。如果子树的头头都汇报上来我的部队不一致了，那么上面的头就更不可能一致了。只有左队长 和 右队长 都说一致，然后头在用自己的值分别和左队长右队长比，还一致就可向上汇报一致了。
int cnt = 0;
public int countUnivalSubtrees(TreeNode root) {
help(root);
return cnt;
}
private boolean help(TreeNode cur){
if(cur == null) return true;
boolean left = help(cur.left);
boolean right = help(cur.right);
if(!left || !right) return false;
int leftv = cur.val;
if(cur.left != null) leftv = cur.left.val;
int rightv = cur.val;
if(cur.right != null) rightv = cur.right.val;
boolean res = cur.val == leftv && leftv == rightv;
if(res) cnt++;
return res;
}
