http://www.lintcode.com/zh-cn/problem/balanced-binary-tree/
注意点: 1 在比较子树之间是否是平衡二叉树时,需要用的树的最大深度,最大深度差<1就是平衡。
2 3 即,左右子树都平衡且最大深度差<1,则是平衡二叉树。
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答案中用-1作为标记,表示树是非平衡二叉树,减少了代码嵌套调用,可以学习。
1 public boolean isBalanced(TreeNode root) { 2 if(root == null) return true; 3 if(isBalanced(root.left) && isBalanced(root.right) && Math.abs(getDep(root.left) - getDep(root.right)) <= 1) return true; 4 return false; 5 } 6 private int getDep(TreeNode root){ 7 if(root == null) return 0; 8 int left = getDep(root.left); 9 int right = getDep(root.right); 10 return Math.max(left,right) + 1; 11 } 12 //--------------------答案 13 public boolean isBalanced(TreeNode root) { 14 // write your code here 15 return maxDepth(root) != -1; 16 } 17 public int maxDepth(TreeNode root) { 18 if(root == null) { 19 return 0; 20 } 21 int left = maxDepth(root.left); 22 int right = maxDepth(root.right); 23 if(left == -1 || right == -1 || Math.abs(right - left) > 1) { 24 return -1; 25 } 26 return Math.max(left,right) + 1; 27 }
当-1做标记不足以完全表示结果时,定义一个新类来表示。
1 class ResultType { 2 boolean isBalance; 3 int maxDep; 4 public ResultType(boolean isBalance, int maxDep){ 5 this.isBalance = isBalance; 6 this.maxDep = maxDep; 7 } 8 } 9 private ResultType helper(TreeNode root) { 10 if(root == null) return new ResultType(true, 0); 11 ResultType left = helper(root.left); 12 ResultType right = helper(root.right); 13 if(!left.isBalance || !right.isBalance || Math.abs(left.maxDep - right.maxDep) > 1) return new ResultType(false, -1); 14 return new ResultType(true,Math.max(left.maxDep, right.maxDep) + 1); 15 } 16 public boolean isBalanced(TreeNode root) { 17 return helper(root).isBalance; 18 }
1 public boolean isBalanced(TreeNode root) { 2 if(root == null) return true; 3 return helper(root, 0) != -1; 4 5 } 6 private int helper(TreeNode root, int dep) { 7 if(root == null) return dep; 8 dep++; 9 int left = helper(root.left, dep); 10 int right = helper(root.right, dep); 11 if(left == -1 || right == -1 || Math.abs(left - right) > 1) { 12 return -1; 13 } 14 return Math.max(left,right) + dep; 15 } 16 //上面代码错误,不能传dep,dep描述的是二叉树的最大深度,而并不是子树的深度。 17 public boolean isBalanced(TreeNode root) { 18 return helper(root) != -1; 19 } 20 private int helper(TreeNode root) { 21 if(root == null) return 0; 22 int left = helper(root.left); 23 int right = helper(root.right); 24 if(left == -1 || right == -1 || Math.abs(left - right) > 1) { 25 return -1; 26 } 27 return Math.max(left,right) + 1; 28 }