1. 设计一个支持 push ，pop ，top 操作，并能在常数时间内检索到最小元素的栈
//leetcode 155 思路：用另一个栈存放最小值
class MinStack {
Deque<Integer> stack ;
Deque<Integer> min_stack ;
/** initialize your data structure here. */
public MinStack() {
stack = new LinkedList<Integer>();
min_stack = new LinkedList<Integer>();
min_stack.addFirst(Integer.MAX_VALUE);
}
public void push(int val) {
stack.addFirst(val);
int min = min_stack.peekFirst();
if(val<min){
min_stack.addFirst(val);
}else{
min_stack.addFirst(min);
}
}
public void pop() {
stack.removeFirst();
min_stack.removeFirst();
}
public int top() {
return stack.peekFirst();
}
public int getMin() {
return min_stack.peekFirst();
}
}
2. 逆波兰表达式求值
// leetcode 150
class Solution {
public int evalRPN(String[] tokens) {
String[] arr = new String[]{"+","-","*","/"};
List<String> operations = Arrays.asList(arr);
Deque<Integer> stack = new LinkedList<Integer>();
int c = 0;
for(String str: tokens){
if(operations.contains(str)){
int a = stack.removeFirst();
int b = stack.removeFirst();
if(str.equals("+")){
c = b + a;
}else if(str.equals("-")){
c = b - a;
}else if(str.equals("*")){
c = b * a;
}else if(str.equals("/")){
c = b / a;
}
stack.addFirst(c);
}else{
stack.addFirst(Integer.parseInt(str));
}
}
return stack.peekFirst();
}
}
3. 字符串解码
// leetcode 394
class Solution { public String decodeString(String s) { int length = s.length(); Deque<String> stack = new LinkedList<String>(); int i = 0; while(i<length){ char c = s.charAt(i); // 从栈中弹出元素开始拼接 if(c==']'){ String temp = stack.pollFirst(); String joint_str = ""; do{ joint_str = temp + joint_str; temp = stack.pollFirst(); }while(!temp.equals("[")); int num = Integer.parseInt(stack.pollFirst()); String temp_result = ""; for(int j=0;j<num;j++){ temp_result += joint_str; } stack.offerFirst(temp_result); i++; }else if(Character.isDigit(c)){ //number String str_value = String.valueOf(c); String joint_str = ""; do{ joint_str += str_value; i += 1; c = s.charAt(i); str_value = String.valueOf(c); }while(!str_value.equals("[")); stack.offerFirst(joint_str); }else{ String stack_value = String.valueOf(c); stack.offerFirst(stack_value); i++; } } String result = ""; String poll_value = stack.pollFirst(); do{ result = poll_value + result; poll_value = stack.pollFirst(); }while(poll_value!=null); return result; }}
4. 克隆图
// leetcode 133
//方法一：广度优先遍历
class Solution {
public Node cloneGraph(Node node) {
if(node==null){
return node;
}
Map<Node,Node> map = new HashMap<>();
map.put(node, new Node(node.val, new ArrayList<Node>()));
Queue<Node> queue = new LinkedList<>();
queue.offer(node);
while(!queue.isEmpty()){
Node temp_node = queue.poll();
for(Node neighbor : temp_node.neighbors){
if(map.get(neighbor)==null){
map.put(neighbor, new Node(neighbor.val, new ArrayList<Node>()));
queue.offer(neighbor);
}
map.get(temp_node).neighbors.add(map.get(neighbor));
}
}
return map.get(node);
}
}
//方法二：深度优先遍历
class Solution {
Map<Node,Node> map = new HashMap<>();
public Node cloneGraph(Node node) {
if(node == null){
return null;
}
if(map.get(node)!=null){
return map.get(node);
}
Node cloneNode = new Node(node.val, new ArrayList<Node>());
map.put(node, cloneNode);
for(Node neighbor:node.neighbors){
cloneNode.neighbors.add(cloneGraph(neighbor));
}
return map.get(node);
}
}
5. 岛屿数量
# leetcode 200
# 方法一：深度优先
class Solution { public int numIslands(char[][] grid) { if(grid==null || grid.length==0){ return 0; } int result = 0; int row_len = grid.length; int col_len = grid[0].length; for(int row=0;row<row_len;row++){ for(int col=0;col<col_len;col++){ if(grid[row][col] == '1'){ result += 1; dfs(grid,row,col); } } } return result; } public void dfs(char[][] grid, int row, int col){ int row_len = grid.length; int col_len = grid[0].length; if(row<0 || row>=row_len || col<0 ||col>=col_len || grid[row][col]=='0'){ return; } grid[row][col] = '0'; dfs(grid, row-1, col); dfs(grid, row+1, col); dfs(grid, row, col-1); dfs(grid, row, col+1); }}
# 方法二：广度优先class Solution { public int numIslands(char[][] grid) { if(grid==null || grid.length==0){ return 0; } int result = 0; int row_len = grid.length; int col_len = grid[0].length; for(int row=0;row<row_len;row++){ for(int col=0;col<col_len;col++){ if(grid[row][col] == '1'){ result += 1; grid[row][col] = '0'; Queue<Integer> queue = new LinkedList<>(); queue.offer(row*col_len+col); while(!queue.isEmpty()){ int value = queue.poll(); int r = value/col_len; int c = value%col_len; if(r-1>=0 && grid[r-1][c]=='1'){ grid[r-1][c] = '0'; queue.offer(col_len*(r-1)+c); } if(r+1<row_len && grid[r+1][c]=='1'){ grid[r+1][c] = '0'; queue.offer(col_len*(r+1)+c); } if(c-1>=0 && grid[r][c-1]=='1'){ grid[r][c-1] = '0'; queue.offer(col_len*r+c-1); } if(c+1<col_len && grid[r][c+1]=='1'){ grid[r][c+1] = '0'; queue.offer(col_len*r+c+1); } } } } } return result; }}
