转载声明：文章来源https://blog.csdn.net/wayne566/article/details/79106372
列举了二叉树的前序、中序、后序的递归和非递归遍历方法，以及层次遍历、分层输出的层次遍历方法。
举例如下：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 | import java.util.LinkedList;import java.util.List;import java.util.Queue;import java.util.Stack;public class Main { public static void main(String[] args) { //构造树结构测试用 TreeNode a = new TreeNode(1); TreeNode b = new TreeNode(2); TreeNode c = new TreeNode(3); TreeNode d = new TreeNode(4); TreeNode e = new TreeNode(5); TreeNode f = new TreeNode(6); TreeNode g = new TreeNode(7); a.left = b; a.right = c; b.right = d; c.left = e; c.right = f; f.left = g; System.out.print("recursivePreOrder: "); recursivePreOrder(a); System.out.print('\n' + "recursiveInOrder: "); recursiveInOrder(a); System.out.print('\n' + "recursivePostOrder: "); recursivePostOrder(a); System.out.print('\n' + "iterativePreOrder: "); iterativePreOrder(a); System.out.print('\n' + "iterativePreOrder_2: "); iterativePreOrder_2(a); System.out.print('\n' + "iterativeInOrder: "); iterativeInOrder(a); System.out.print('\n' + "iterativePostOrder: "); iterativePostOrder(a); System.out.print('\n' + "iterativePostOrder_2: "); iterativePostOrder_2(a); System.out.print('\n' + "iterativePostOrder_3: "); iterativePostOrder_3(a); System.out.print('\n' + "iterativeLevelOrder: "); iterativeLevelOrder(a); System.out.print('\n' + "iterativeLevelOrder_2: " + '\n'); iterativeLevelOrder_2(a); System.out.print('\n' + "recursiveLevelOrder: "); recurLevelOrder(a); System.out.print('\n' + "recursiveLevelOrderBottom: " + '\n'); List<List<Integer>> lists = recursiveLevelOrderBottom(a); for (List<Integer> list : lists) { for (int p : list) { System.out.print(p + " "); } System.out.println(); } } public static void visit(TreeNode p) { System.out.print(p.val + " "); } //**********递归的先序遍历********** public static void recursivePreOrder(TreeNode p) { if (p == null) return; visit(p); recursivePreOrder(p.left); recursivePreOrder(p.right); } //**********递归的中序遍历********** public static void recursiveInOrder(TreeNode p) { if (p == null) return; recursiveInOrder(p.left); visit(p); recursiveInOrder(p.right); } //**********递归的后序遍历********** public static void recursivePostOrder(TreeNode p) { if (p == null) return; recursivePostOrder(p.left); recursivePostOrder(p.right); visit(p); } //**********非递归的先序遍历********** //手算的思想，先变访问边找，找到最左下方的，然后向上再向访问右边的 public static void iterativePreOrder(TreeNode p) { if (p == null) return; Stack<TreeNode> stack = new Stack<TreeNode>(); while (!stack.empty() || p != null) { while (p != null) { visit(p); stack.push(p); p = p.left; } p = stack.pop(); p = p.right; } } //**********非递归的先序遍历********** //栈的思想，按层次倒着进栈，利用后进先出解决顺序问题 public static void iterativePreOrder_2(TreeNode p) { if (p == null) return; Stack<TreeNode> stack = new Stack<TreeNode>(); stack.push(p); while (!stack.empty()) { p = stack.pop(); visit(p); if (p.right != null) stack.push(p.right); if (p.left != null) stack.push(p.left); } } //**********非递归的中序遍历********** public static void iterativeInOrder(TreeNode p) { if (p == null) return; Stack<TreeNode> stack = new Stack<TreeNode>(); while (!stack.empty() || p != null) { while (p != null) { stack.push(p); p = p.left; } p = stack.pop(); visit(p); p = p.right; } } //**********非递归的后序遍历********** //注意prev的作用 public static void iterativePostOrder(TreeNode p) { if (p == null) return; Stack<TreeNode> stack = new Stack<TreeNode>(); TreeNode prev = p; while (!stack.empty() || p != null) { while (p != null) { stack.push(p); p = p.left; } p = stack.peek().right; if (p == null || p == prev) { //若栈顶节点的右节点为空或者已经visit过，则按顺序应该访问栈顶节点 p = stack.pop(); visit(p); //prev用来标记已经visit过这个节点 prev = p; p = null; } } } //**********非递归的后序遍历********** //和上一种方法思想类似 public static void iterativePostOrder_2(TreeNode p) { if (p == null) return; Stack<TreeNode> stack = new Stack<TreeNode>(); TreeNode prev = p; while (p != null) { while (p.left != null) { stack.push(p); p = p.left; } while (p != null && (p.right == null || p.right == prev)) { visit(p); prev = p; if (stack.empty()) return; p = stack.pop(); } stack.push(p); p = p.right; } } //**********非递归的后序遍历********** //双栈法，易于理解 public static void iterativePostOrder_3(TreeNode p) { if (p == null) return; Stack<TreeNode> stack = new Stack<TreeNode>(); Stack<TreeNode> result = new Stack<TreeNode>(); while (!stack.empty() || p != null) { while (p != null) { stack.push(p); result.push(p); p = p.right; } if (!stack.empty()) p = stack.pop().left; } while (!result.empty()) { p = result.pop(); visit(p); } } //**********非递归的层次遍历********** public static void iterativeLevelOrder(TreeNode p) { if (p == null) return; LinkedList<TreeNode> queue = new LinkedList<TreeNode>(); queue.offer(p); while (!queue.isEmpty()) { p = queue.poll(); if (p.left != null) queue.offer(p.left); if (p.right != null) queue.offer(p.right); visit(p); } } //**********非递归的分层输出的层次遍历********** public static void iterativeLevelOrder_1(TreeNode p) { if (p == null) return; Queue<TreeNode> queue = new LinkedList<TreeNode>(); queue.offer(p); while (!queue.isEmpty()) { int levelNum = queue.size(); for (int i = 0; i < levelNum; i++) { p = queue.poll(); if (p.left != null) queue.offer(p.left); if (p.right != null) queue.offer(p.right); visit(p); } System.out.println(); } } //**********非递归的分层输出的层次遍历********** //维护两个int，代表上一层和下一层的节点数量，上一层遍历结束之后lineUp = lineDown; lineDown = 0; public static void iterativeLevelOrder_2(TreeNode p) { if (p == null) return; LinkedList<TreeNode> queue = new LinkedList<TreeNode>(); int lineUp = 1, lineDown = 0; queue.offer(p); while (!queue.isEmpty()) { p = queue.poll(); visit(p); if (p.left != null){ queue.offer(p.left); lineDown++; } if (p.right != null){ queue.offer(p.right); lineDown++; } if (--lineUp == 0) { lineUp = lineDown; lineDown = 0; System.out.println(); } } } //**********递归的层次遍历访问********** public static void recurLevelOrder(TreeNode root) { if (root == null) return; int depth = maxDepth(root); //如果要倒序访问只需修改此处顺序 for (int i = 1; i <= depth; i++) visitNodeAtDepth(root, i); } //访问特定层的节点 public static void visitNodeAtDepth(TreeNode p, int depth) { if (p == null || depth < 1) return; //因为要按顺序访问（打印），所以要规定必须到某一层才能visit if (depth == 1) { visit(p); return; } //每次都要遍历depth之上的所有层 visitNodeAtDepth(p.left, depth - 1); visitNodeAtDepth(p.right, depth - 1); } //得到树的层数 public static int maxDepth(TreeNode root) { if (root == null) return 0; return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1; } //**********递归的倒序层次遍历并保存结果至list********** //LeetCode107 //之所以用LinkedList是因为有addFirst()方法，可以逆序保存 public static List<List<Integer>> recursiveLevelOrderBottom(TreeNode root) { LinkedList<List<Integer>> lists = new LinkedList<List<Integer>>(); addToList(lists, root, 1); return lists; } //将depth层的p节点保存至list public static void addToList(LinkedList<List<Integer>> lists, TreeNode p, int depth) { if (p == null) return; if (lists.size() < depth) lists.addFirst(new LinkedList<Integer>()); //由于不用输出只是保存，可以使用get控制保存在哪一层，所以不用规定层数 lists.get(lists.size() - depth).add(p.val); addToList(lists, p.left, depth + 1); addToList(lists, p.right, depth + 1); }} |
运行结果：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 | recursivePreOrder: 1 2 4 3 5 6 7 recursiveInOrder: 2 4 1 5 3 7 6 recursivePostOrder: 4 2 5 7 6 3 1 iterativePreOrder: 1 2 4 3 5 6 7 iterativePreOrder_2: 1 2 4 3 5 6 7 iterativeInOrder: 2 4 1 5 3 7 6 iterativePostOrder: 4 2 5 7 6 3 1 iterativePostOrder_2: 4 2 5 7 6 3 1 iterativePostOrder_3: 4 2 5 7 6 3 1 iterativeLevelOrder: 1 2 3 4 5 6 7 iterativeLevelOrder_2: 1 2 3 4 5 6 7 recursiveLevelOrder: 1 2 3 4 5 6 7 recursiveLevelOrderBottom: 7 4 5 6 2 3 1 |
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